You will edit keyframes, i.e., key positions or appearances of your model at specific times (frames) in your animation.
Editing keyframes means that you will apply deformations to your model (move it to a different location, modify its scale, move vertices, or bend it) and you will specify the time at which this new appearance of your model will be displayed. Between two key frames, the Key Framer will use linear interpolation to compute all intermediate steps of the shape of your model, generating this way an animation sequence.