Flip Up is the term used to describe the property that something can be folded up. With airsoft guns, this almost always refers to so-called flip-up sights. So foldable sights. These are simple sights (consisting of front and rear sight) which are mostly used as backup sights.
These can either be integrated into the weapon or removable. A mixture is also possible. For example the H&K 417 Sniper has a removable flip up rear sight. The front sight is integrated into the hand guard.