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.
HM01 is none other than the move known as Cut. HM Cut can be obtained from the SS Anne Captain while the cruise ship is still docked in the Vermilion City Harbor. With HM Cut, you will be able to challenge the Vermilion City Gym, access to Route 9, and challenge the Celadon City Gym. You will even need HM Cut to open the Dotted Hole Entrance much later in the Post Game.