How do you fix cracks on a concrete garage floor?

If you're talking about a crack, you can use an epoxy filler. ...or if it's not too wide, silicone calk will work fine. If You're talking about a flaw (like an air pocket) from when the slab was poured, you can use mortar or furnace cement, something that's sticky and has good adhesion. You can check with the folks at your local hardware or home improvement store. They'll recommend something.