Rolled Weatherseal: this type of bottom seal is the most common on residential garage doors. It may be time to replace it if it is cracked, torn, out of shape, or has missing pieces. Replacing your garage door weather seal will help keep the elements out of your garage, and will block out all daylight from shining through.
The rolled weatherseal is formed into a u-shape, fit into the metal retainer, and pulled across the length of the garage door. Any excess at the ends should be folded into itself instead of cut. Pro-Tip: spray a general-purpose lubricant onto the grooves of the retainer prior to installation to make pulling it across much easier.
If the existing aluminum retainer is severely weathered, bent, or otherwise damaged- then buy the combo weatherseal and retainer all together.