It's really useful if you started using Facebook back when it was focused on college students -- I joined in 2004, and I found posts I had totally forgotten about. Now you can go through and delete all of those things at once, instead of one by one.
Here's how to do that.
Facebook will show you all of your old posts, activity (like when you started new jobs), likes you've made on other posts and more. If you want, you can also tap the "Filters" button at the top of the page to see different activity by year, category, or interactions with specific people.
Check each box next to each item to delete or archive it.
If you archive, the posts will be removed but you'll have a record for yourself. If you delete them, they're gone forever.