Within Pokemon FireRed, there are seven different HMs (Hidden Machines) to obtain during your journey. HMs work a lot like TMs in that they are moves that can be taught to your Pokemon; however, unlike TMs, HMs last forever and even can be used in the overworld. Obtaining all of the HMs will also be vital, since you won't be able to complete the entire Main Story & Post Game without them.
HM02 is none other than the move known as Fly. HM Fly can be obtained from the person within the house found on the far northern side of Route 16, only accessible by having HM Cut and the Poke Flute. With HM Fly, you will be able to fly to any town you previously visited, saving loads of time compared to backtracking on foot.