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.
HM03 is none other than the move known as Surf. HM Surf can be obtained as a gift for reaching the house at the far end of the Safari Zone in Area 4 before the step count is reached. HM Surf will allow you to travel across waters, which you will need in order to reach the Cinnabar Island Gym and the Power Plant (to find the Legendary Pokemon known as Zapdos).