A "patch" is an area where lots of little things grow (or don't grow). For example, in your garden you can have "a strawberry patch". On your head, an area where hair doesn't grow is "a bald patch".
There's also another meaning of "patch", which is a flat piece of something that you use to fix something that's broken. You sew on a "patch" to fix a ripped piece of clothing, for example.