Frame markers are simple colour markers that you can add in the Timeline view. They can be added to any frame, regardless of whether or not it contains a drawing. They can help you organize your scene by marking important frames in your scene.
Contrary to scene markers, frame markers are placed on a specific frame in a specific layer. They can only be added to a single frame, and not to a span of frames.
Frame markers also distinguish themselves from drawing markers in that they do not mark drawings, but frames. They can even be added to frames that do not contain any drawing. They are also only visible in the Timeline view.