A: The two sides and top of your exterior doors are sealed with weather stripping. The bottom is generally sealed with a door sweep. Even with very good doors from reputable manufacturers these will both need to be replaced periodically. The weather stripping becomes compressed and hard, and the door sweep starts to fall apart over time.
There are multiple styles of each and you’ll want to stay with the styles the manufacturer shipped with your door. This is what it was designed for, you just need a new version of that product. Once installed, stand back from your door on a sunny day. No light should be visible around the door. If you can see light, something isn’t installed properly, or the door needs adjustment.