In the passages leading up to these, Peter has referred to putting his readers in remembrance of certain things. He then speaks of his impending departure from this world and again refers to their keeping these things in remembrance. He supports the things which he has spoken by giving evidence of the things that he has personally seen and heard firsthand.
And then he says:
2 Peter 1:19 We have also a more sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day star arise in your hearts:
2 Peter 1:20 Knowing this first, that no prophecy of the scripture is of any private interpretation.
2 Peter 1:21 For the prophecy came not in old time by the will of man: but holy men of God spake [as they were] moved by the Holy Ghost.
Peter says that nothing which he has said should stand on its own, that it is supported by the Old Testament prophets as well.
