What is the "re-seeding"? Why should I use it?

The NFL re-seeds after every round so that the worst-seeded team will always match up against the highest-seeded remaining team in the next round. If you select to re-seed, your league will also reseed.

For example, in a six-team playoff tournament with the top two teams getting byes, if the #3 seed faced the #6 seed in the first round and the #6 seed won, they would automatically face the #1 seed. If the #3 seed won, they would face the #2 seed with the winner of the #4 vs. #5 matchup facing the #1 seed in the 2^nd round. (This is exactly how the NFL does it.)

If you don't re-seed, the playoff bracket will remain "fixed" throughout, just like it does in the NBA and NHL playoffs.