What is sacroiliac joint syndrome?

Sacroiliac joint syndrome of SIJ syndrome for short, generally refers to pain in the SIJ region that is caused by abnormal motion in the SIJ (can be too much or too little).

We refer to it as a “syndrome” as there can be multiple reasons for pain in this region. Not only are the joint surfaces a possible pain generator, but there are multiple muscles and ligaments that surround and attach to the SIJ at the front and back. These may also contribute to the pain pattern.

I can only assume that your diagnosis is correct, however most people complain of pain in the lower back, occasional referred pain into the buttock, groin or hip. Pain does not usually travel past the knee.

In practice, we have a special interest in the role that the SIJ has in low back and spinal function. Every body is unique and requires a tailored approach. I suggest that you listen to your health care professional and follow their lead. In my experience, SIJ syndrome responds well to manual therapies.

The sacroiliac joint or SI joint is a joint formed between the triangular bone at the base of the spine (sacrum) and a part of the pelvis called the ilium. Our bodies have 2 sacroiliac joints, but commonly one of the joints is causing the pain.  The main symptoms associated with sacroiliac joint syndrome are sharp or aching pain on one side of the low back, over either SI joint. The pain may also refer into the groin and/or into the thigh (usually above the knee) on the same side as the affected joint. SI joint pain is usually increased with sitting for long periods and patients find it difficult arising from a seated position.  As suggested above, Sacroiliac joint syndrome typically responds well to manual therapy and tailored exercises to address any muscular imbalance or instability. There are a number of conditions which may mimic the symptoms of Sacroiliac joint syndrome, so it is important to consult your health care professional for an accurate diagnosis and treatment.

Danny Diab - Chiropractor Narellan

That is a very broad term, which could cover a multitude of things. People often attribute pain in the low back to the sacroiliac joint. But pain in this area is not necessarily coming from the sacroiliac joint. Sacroiliac joint syndrome is usually some sort of instability in the sacroiliac joint.

It sounds like lack of a correct diagnosis. 
The term syndrome refers to a collection of symptoms and is not a diagnosis. 
In my experience majority of sacro-iliac problems are actually lumbar disc problems.
When we resolve the lumbar disc issue (with the McKenzie method which focuses on identifying a particular exercise you can do to relieve your own pain), the sacro-iliac symptoms almost always resolve.
30-40% of disc problems bulge more to one side, and require lateral (movements that address one side) mckenzie exercises as part of their resolution. Frequently these asymetrical disc problems cause a pelvic rotation (which many health professionals fail to correctly diagnosis, and mention things like “one leg is longer” or “your hip is out”), which then causes secondary impingement of the sacro-iliac joint.
The answer is usually simple:
Sort of the disc problem (within 1-2 sessions a McKenzie Qualfied therapist can find the correct exercise, called the Direciton Preference), and the SIJ pain resolves with it. 

Joel Laing's answer above will help you out alot.  I am a McKenzie trained physio like Joel. 

Here's my summary:
Most pain located in the back is coming from the back. 
SIJ pain is rare in my experience.  Typically, women and especially when pregnant. 

What can help?
See a McKenzie trained physio who will rule your back in or out as a source of pain.  Typically, McKenzie physio's see you around 4-5 times and will show you how to stop the pain from coming back. 

The current literature suggests that sacroiliac joint pain is suprisingly common. "Sacroiliac joint (SIJ) pain is an underappreciated source of mechanical low back pain, affecting between 15 and 30% of individuals with chronic, nonradicular pain"

Sacroiliac joint pain: a comprehensive review of epidemiology, diagnosis and treatment.Cohen SP, Chen Y, Neufeld NJ. Expert Rev Neurother. 2013 Jan;13(1):99-116

A systematic evaluation of prevalence and diagnostic accuracy of sacroiliac joint interventions. Simopoulos TT, Manchikanti L, Singh V, Gupta S, Hameed H, Diwan S, Cohen SP.Pain Physician. 2012 May-Jun;15(3):E305-44.

the origin of sacro iliac problems is a vast discussion…the nervous system is just that a system and it controls the entire body it is the first organ system to develop in utero. the control of the muscle systems that move the bones are controlled by this system and what has been overlooked in all of the above answers to this question is the role that the TMJ (Jaw) plays in the normal function of the sacro iliac joints there position and their function. the other systems in the body need to be balanced for all systems to work as one. the body follows the head, each are of the body has an area of compensationa dn corelation as above so below these are natures laws so we must look holistically at the human frame and anatomy, the right brain controls right body and left brain the right body if you really want to understand how the body works then it is best to study and understand neural organization technique as most structural  human conditions can be addressed and in many cases resolved with this technique.