Something which is said to have 'seen better days' is considered well used, old or in generally bad condition. This is an idiomatic expression which is often used as a polite way of saying something needs to be replaced or looks old.

1. That jacket you're always wearing has seen better days. Why don't you get yourself a new one?
2. Not only is my fridge on its last legs, my car has seen better days too. I think I'd better start saving up some money!

1. The building we're working in has seen better days. It's not good for staff to have to work in such a cramped, dilapidated environment.
2. I think we need to innovate this product. The design is dated and has seen better days.