A frame is the smallest unit in a video. One frame is a still image. With multiple frames moving together, an animation is created.
For example, in the picture below, there are 7 images(each image is a frame). By playing the 7 frames continuously, viewers can see a video in which a rectangle gradually turns into a circle.
The concept of keyframes can be seen in animation production, post-production, and video compression.
In post-production, at least two keyframes are needed to make an animation. According to the parameters you set in the first and second keyframe, computers can analyze the parameters and calculate the change between the two sets of parameters to present an animation.