In media production, a key frame is a location on a timeline which marks the beginning or end of a transition. It holds special information that defines where a transition should start or stop. The intermediate frames are interpolated over time between those definitions to create the illusion of motion. In computer animation, like 3D animation or non-linear video editing, this interpolation is performed mathematically by the CPU.
In the following screenshot from Adobe After Effects, the keyframes are highlighted with red arrows. Each of the keyframes defines a position of the animated logo (first on the left, and then one second later on the right). When the animation is played, the logo transitions smoothly from the start position to the end position.
Key frames can be used in any time-based media production process where transitions are defined, including audio production and high-end digital video compression.