Marked frames where animators set position and parameter values — software interpolates in-between frames. Speed over manual work.
You set one frame, then the next — the computer interpolates everything in between. That's the principle, and it revolutionized animation and VFX because it replaces craft with intelligence. Keyframes are the anchor points of your movement, your control points in space and time. Without them, you'd have to draw or model every single frame by hand; with them, you only describe the critical moments, and the software does the rest.
In practice, it works like this: You position a 3D object, a camera, or an effect property at frame 1. You set a keyframe. Then you jump to frame 30 and move the object to a new position — new keyframe. The computer now automatically calculates the 28 frames in between through linear or spline interpolation. This saves time but also guarantees smoothness if the curves are set correctly. In motion capture, you would proceed similarly: skeleton positions are saved as keyframes, and the animation then fills in between the keyframes.
The trick lies in controlling the transitions. You can flatten keyframe curves in the graph editor view, make them steep, and work with easing functions (ease-in, ease-out) — this shapes the character of the movement. An abrupt cut between keyframes looks robotic; a smooth spline curve feels organic. That's why animators don't just work with the positions themselves, but with the tangents between the frames. This works the same way for rotations or color gradients: keyframe the rotation, keyframe the color, let the computer calculate the intermediate steps.
In the classic frame-by-frame workflow (rotoscoping, traditional animation), you didn't need anyone for keyframes — only for the in-betweeners. Today, every motion design software, every 3D engine is equipped with it. In compositing tools like Nuke or After Effects, you set keyframes for every parameter: position, scale, opacity, effect intensity. The timeline becomes your workspace. And because keyframes are calculable, they can also be scripted, duplicated, linked with expressions — automation at a high level. This is why modern VFX pipelines are so efficient.