From Halopedia, the Halo wiki
"Double Melee" refers to three button combos present in Halo 2, all of which have the same effect: they allow a player to melee twice, with no delay between the attacks. The three combos are B+X+B, B+Y+B, and B+L+B, the last of which also works in Halo: Combat Evolved.
When a player performs the combo, they start by meleeing an opponent. As soon as the melee hits and does damage, the attacker then presses X, which cancels the rest of the melee animation and nullifies the remaining delay. (If the attacker possesses a secondary weapon, they can also press Y; switching weapons also cancels a melee animation.) The attacker can then press B again without waiting for the now-canceled melee delay to elapse.
If the player has a secondary weapon, they can Y between melees to switch and melee with your secondary, or double-tap Y to melee with their primary weapon again. This is useful if carrying an Energy Sword or Brute Shot, both of which have much stronger melee attacks than other weapons.
The B+L+B combo is also usable in Halo: Combat Evolved. As soon as your melee has hit your opponent and the damage is done, you can cancel the rest of the animation by pressing the left trigger to throw a grenade; as with X and Y in Halo 2, this cancels the remainder of the melee animation. The Plasma Grenade is preferred when doing this. Also, if done correctly, you can continuously melee your opponent by pressing chaining B+X rapidly and repeatedly, though if done too fast, the combo won't work.
A variation of this combo can also be performed in Halo 3 by pressing B+RB+RB+B. By pressing the reload button twice, you will actually start to reload your gun and then cancel the reload animation, allowing you to quickly melee again. Note: This only works with a less than full clip, and reserve ammo. In other words, if you cannot reload, it will not work.
Another variation of this combo can also be performed in Halo: Reach Beta by pressing RB+X+RB.
 (Includes other combos as well.)