From Halopedia, the Halo wiki
Telefragging is an unknown killing force in multiplayer gameplay of Halo: Combat Evolved and Halo 2 that kills players who block teleportation receivers when another player attempts to use the sender. It is a rare occurrence, and usually must be performed deliberately. It seems to be similar to the The Guardians.
This is not a glitch and is an intentional part of the game that is designed to prevent both the blocking of teleporters, and players teleporting directly onto another player. Telefragging can be prevented either by the player standing on the receiver and moving off of the receiver, or by the player going through the sender backing away from the sender.
Halo: Combat Evolved
In Halo: Combat Evolved, a player must go through any teleporter and remain standing still at the exit of the teleporter. If another player tries to go through the same teleporter, they will not be able to go through. However, the first player's screen will start to go white and their controller will start to shake. Their body will then start to go sprawling away from the teleporter, and, if said player dies, the game will say that Player 2 "telefragged" Player 1. In Halo Trial if said player survives, they will see a message in top left corner saying "You were telefragged!".
In Halo 2, by parking any vehicle or standing in a teleporter you can block it. When another player attempts to use the teleporter, they will not be able to teleport and will receive a message "Teleporter is blocked", If a player is standing on the receiver their screen will gradually turn white, similar to the effect in Halo: Combat Evolved, and when the player's screen has gone completely white they will die and it will be attributed as a suicide.