Frame rate (measured in frames per second, or FPS) is how many individual image frames a video displays per second. Higher frame rates produce smoother, more fluid motion. Lower frame rates produce a cinematic, slightly "choppy" look. Common frame rates: 24fps (cinema standard — the "filmic" look), 25fps (European broadcast PAL standard), 30fps (US broadcast NTSC, YouTube default), 60fps (sports, gaming, slow-motion), 120fps (high-speed slow-motion).
Frame rate significantly affects file size: a 60fps video contains twice as many frames as 30fps, and will be roughly 2x larger at the same resolution and bitrate. For compression, the encoder can exploit temporal redundancy between frames — consecutive frames in a slow-moving video are very similar, so the encoder stores only the differences. High-motion content (sports) has less frame-to-frame similarity and compresses less efficiently.
Matching frame rate to use case: 24fps for cinematic narrative video, 30fps for general YouTube/social content, 60fps for gaming videos and sports, 120fps+ for slow-motion effects. Converting a 60fps video to 30fps reduces file size significantly with no quality loss for non-motion content — FileCurve's video compressor maintains original frame rate.