Back when people used film to takes pictures, prints came back from the processor with negatives.
These negatives are a backup to your printed photos and can be saved in case of damage to the original prints.
The most important to do with negatives: Store them separate from your prints. If something tragic were to happen to those prints (think: house fire or water pipe break), the negatives could be used to recreate the print if they are stored somewhere else.
If you’re sorting through a mountain of family photos and come across a bunch of negatives, set them aside. The plastic sleeves in which some negatives come from the processor are likely photo-safe plastic (i.e., polyethylene) so it’s fine to keep them in those sleeves or — if the negatives are loose — the original photo envelope. Negatives don’t have to be sorted and edited as meticulously as a print collection unless or until it becomes necessary, as in the case of one of the aforementioned tragedies.
Store your negatives in the same way you would store your prints. Use photo-safe materials, and keep them in a cool, dry place (i.e., not the basement or attic) or better yet, the house of a good friend or relative.