From Halopedia, the Halo wiki
Flag Taken is a multiplayer medal awarded for taking an opponent's flag from their base in Halo 2 and selected games in Halo: The Master Chief Collection (Halo: Combat Evolved, Halo 2, and Halo 2: Anniversary multiplayer). It is available in all multiplayer CTF gametypes. It is represented by a yellow triangle with a flag. In Halo 2, this medal is not observed during gameplay and can only be viewed in the post game lobby.
The medal has a Bungie difficulty rating of 4 (out of 10).