Video is a sequence of still frames shown quickly enough that the eye reads them as motion. Everything about how video is stored and processed follows from that simple structure. Three ideas describe any video signal: resolution, frame rate, and aspect ratio.
Resolution
Resolution is the number of pixels in each frame, usually written as width by height. Common names are 720p (1280 by 720), 1080p (1920 by 1080), and 4K or Ultra HD (3840 by 2160). More pixels mean finer detail, and also more data: a higher-resolution frame takes more memory to store, more time to process, and more bandwidth to stream.
Frame rate
Frame rate is how many frames are shown per second. Established values include 24 fps, long associated with film, 30 fps, common in television-style content, and 60 fps, often used where motion needs to look smooth. Higher frame rates produce smoother motion and, like resolution, multiply the data a pipeline must handle.
Progressive versus interlaced
Video can be progressive, where each frame is a complete picture, or interlaced, where each frame is split into alternating rows shown in two passes. Interlacing was a trick of older analog television systems to make motion smoother within limited bandwidth. Modern digital video is almost entirely progressive, and interlaced sources usually need deinterlacing before further processing.
Aspect ratio
Aspect ratio is the shape of the frame: the ratio of width to height. The 16:9 shape is standard on modern displays, while 4:3 belongs to an earlier television era. Mismatched aspect ratios produce letterbox bars or stretched pictures, so keeping the shape consistent through a project is a basic but important discipline.
Why these choices are trade-offs
Resolution and frame rate are always a negotiation between fidelity and cost in storage, bandwidth, and processing time. A ten-second clip at high resolution and frame rate holds far more data than the same clip at standard settings. Choosing sensibly for the delivery target, rather than always choosing the maximum, is a core video processing skill.