The word patch has several meanings:

1. A piece of material used to cover a hole or worn spot. When your jeans rip at the knee, you might sew on a patch to cover the tear. Sailors once wore eye patches to protect injured eyes. A patch of fabric can fix something broken or add decoration, like the colorful patches scouts earn and display on their uniforms.
2. A small area that differs from what surrounds it. A patch of blue sky might appear through storm clouds. You might find a patch of strawberries growing in the corner of a garden, or notice a patch of ice on an otherwise clear sidewalk. When something appears “in patches,” it shows up here and there rather than everywhere.
3. To repair or fix something, often temporarily. Software companies patch programs to fix bugs or security problems. A doctor might patch up a patient's injuries in an emergency room. You can patch a bicycle tire, patch a friendship after an argument, or patch together a solution from whatever materials you have available.

The phrase “not a patch on” means something doesn't compare well to something else, like saying your substitute teacher isn't a patch on your regular teacher.