From DoomWiki.org

Flight is a feature in Heretic, Hexen and some source ports where the player can move freely in all three dimensions and isn't subjected to gravity. In the two Raven Software games, this is a power-up granted by using the Wings of Wrath. It may also be implemented merely as a cheat mode.

Flight is generally combined with freelook so that aiming up or down and moving forward will also move up or down, as appropriate; however separate keys for direct vertical movement are also available, as well as one for landing.

In multiplayer-oriented ports, spectating players may also fly to find a good vantage point to see the action.

Implementing flight in a source port generally requires to also implement true over/under collision checks, instead of the infinitely vertical actors used by vanilla Doom.