Grenade Reload is a button combo present in Halo 2. The combo allows players to simultaneously shoot and reload a weapon.
If a player has shot a few rounds already, and they want to reload at the same time, then they should press L, and then quickly press X. During the resulting reload animation, the player can continue to fire their weapon while it reloads at the same time.
The combo works because it causes a communication error to occur when the player's actions are sent to the connection host; therefore, it can only be done when a player is not host.