From Halopedia, the Halo wiki
PFR, or Plasma, Frag, Rocket, is the act of throwing a Plasma Grenade, then throwing a Frag Grenade, and last shooting a rocket, consecutively at the same area of ground. One can use this to send himself or herself, another player, or a vehicle flying very far in either Halo: Combat Evolved and Halo 2.
Method (Halo:Combat Evolved and Halo 2)
In order to do this you need customized shielding (Halo: Combat Evolved) or an Overshield (Halo 2). In Halo 2, this can be harder to do since the time to throw a grenade after another is longer than in Halo: Combat Evolved.
There are several ledges and places you can get to by doing this. On the Halo: Combat Evolved multiplayer map Blood Gulch for example, if you do this with 400% shielding on, you can reach the top of the cliff pillar near the blue base. Just make sure your "willing" victim knows when you are going to do it.