The posts in that community will be deleted after that many days. For remote communities this only affects the local copy of each post and the post will still exist on the source instance and other instances (unless they too delete their copies). For local communities the deletion federates so the post is removed everywhere.
PieFed is intended to run for decades and storage costs money so admins can set how long they want to retain content for, on a per-community basis. It's up to them to decide what is worth keeping forever and what should be trimmed.
AFAIK piefed.social has 'forever' for the retention of all local communities and only trims remote communities. In some communities piefed.social only keeps posts for very short periods, like 28 days, because it's the kind of content people look at once and has no lasting value beyond that.