Keyframes are the building blocks of Cut-out animation. Unlike Traditional animation, where every pose is drawn, Cut-out animation uses keyframes to mark major poses (key poses) and lets the software create all the poses in between.
In Harmony, you can create two kinds of keyframes: Motion and Stop-Motion. Motion keyframes behave like the description above; they allow for the automatic creation of drawings between two keyframed poses. Stop-Motion keyframes do not allow for interpolation between keyframes. They are often used to block poses, somewhat similar to what you would see in Traditional Animation.