A shoulder rest attaches to the back of the body to hold the fiddle up under your chin. It could be a sponge or other device that lifts and steadies the fiddle on your shoulder. Not every player uses a shoulder rest but most will.
Fiddles are slippery. If you put a fiddle on your shoulder with no shoulder rest it will slide around and require you to scrunch up your shoulder and your neck to try to pin it in place so that you can play. This is not comfortable for your upper body.
Even if your neck is very short or your shoulders are very big, or some combination (the wrestler musician), the sliding will interfere with your ease in playing.
At the very least, if you don’t want any kind of shoulder rest, purchase one of the small, round red pads that stick to the back of the instrument or are attached by a rubber band. This will allow you to grip and prevent sliding.
Most violinists and fiddlers use a traditional shoulder rest. There are a number of different styles to choose from and they all have slightly different shapes and are adjustable in different ways.
A good start for a beginner is to get a KUN shoulder rest and adjust the height so that along with the violin it fills most of the space between your shoulder and your chin and feels comfortable.
I still call a shoulder rest a KUN sometimes. I use this. It is the most popular shoulder rest, but it is not very adjustable. It usually either feels good or it doesn’t. Fits easily in most violin cases.
If you are having trouble keeping your violin in place this can be the shoulder rest that finally beats the readjustment habit. A bit like super gluing your violin to your shoulder, and highly adjustable.