GOPATH is the root of your workspace. Within it you'll find a directory structure that resembles something like this:
src/github.com/username/project
This helps Go to (automatically) resolve imports with full canonical paths. For example you can do
import "github.com/foo/bar"
in your code. Go would import that package (and potentially git clone from GitHub first) from $GOPATH/src/github.com/foo/bar