Measured in frames per second, frame rate determines how finely motion is sampled. 24fps is the cinema standard and has been since sound arrived; it retains just enough blur between frames to feel filmic. 30fps looks slightly crisper and reads as broadcast. 60fps and above look hyper-real, an effect often described as the soap-opera look.
Higher rates also enable slow motion. Shoot at 120fps and play back at 24 and you get motion at one-fifth speed with smooth, complete frames — far better than software attempting to invent intermediate ones. The cost is light: shorter exposures per frame mean darker images, and high frame rates often force a resolution or crop compromise.
Frame rate should be locked to delivery. Mixing 24 and 30 in one timeline produces judder that no amount of grading hides. The one deliberate exception is high-frame-rate footage shot specifically to be slowed down, which is conformed to the project rate in the edit.