This means suddenly loud. Usually this dynamic is during a soft dynamic such as piano. This dynamic can be compared to an accent, but keeps the loud and hard tone. It is great for showing emotion and sudden picking up of a tune and melody.
The subito forte ("suddenly strong," sf) indicates that the note is to stick out from the preceding and following notes, but Beethoven also seems to intend the accentuation to stay within the general "box" of whatever abiding dynamic is indicated for the passage, such that an sf indication within a piano passage would probably not be sounded at a true forte.
