Here, the frame refers to the edges of your photograph or the edges of the viewfinder of your camera when you are shooting. The advice to fill the frame means to get in close, to make your subject a significant portion of the final photograph.
One of the most effective and simple composition techniques is to fill your frame with your point of interest. Doing so will greatly enhance your digital photographs and eliminate all that background clutter.
Filling the frame is a good technique when there’s a distracting background or you just want to make a statement about the point of interest. When using this technique be sure and set the depth of field so that the whole image is sharp.