of ‘slop’ in the rails and rollers since nothing is ever totally plumb, parallel, or level. Not to mention that the curved portion of the railing at the top of the door have to have enough clearance to allow the door to open and close without hitting the header.
All rails are adjustable within a certain amount. The problem is that you are dealing with a door that could weigh 300-400 pounds suspended 8 feet in the air. If you loosen the wrong bolt too much or try to move something in the wrong direction the whole thing could end up on your head.
There are garage door weather strips that are 3-4 inches wide and can cover any normal gap between the door frame and the door, maybe 3/4 to 1 1/2 inches. Most home stores have them. If your door is looser than that then you really should call in a professional to adjust it or find the problem. It could be nothing more than moving a few bolts.