When installing a garage door, the floor along the line of contact is used as a reference to scribe the bottom of the door face so it can be cut or planed in order to closely match the contour of the floor. A seal is then installed on the bottom edge of the door for a more water resistant fit.
You should be able to find a Do-It-Yourself book with a pictoral description of this process at the library.
If your gap is minimal, a new door bottom seal might do the trick, but the above process is the recommended.