Prewashing your fabric basicly means that you wash your fabric before you start sewing. Why should you do that?
Wash off chemicals
During the making process of fabric, many chemicals are involved. If you don’t wash these off first, by prewashing your fabric, these chemicals can enter your body through your skin when you wear your homemade ,or newly bought, clothes.
For Jersey and other knitted and/or elastic fabrics, another important reason is involved. Shrinking! Jersey can shrink up to 5% when washing it for the first time. You really don’t want that to happen to the clothes you spend so much effort on making them.
When I prewash my fabric I use a short coldwater program (20 minutes on 30°C). I also add some vinegar to help prevent color ‘bleeding’. It will help the colors (paint) to fixate on your fabric. More on using vinegar in your laundry you can find here.