This warning is very instructive. First, Peter assures us that biblical prophecy is "more sure" than even eyewitness testimony (verses 16-18)! When God speaks, whatever He foretells WILL happen! God's Word will not return to Him empty; it will accomplish what God sends it to do (Isaiah 55:11).
The apostle also says we would "do well to heed" it. Prophecy is vital to our growth! It strengthens our faith in God, teaches us how He works, and gives us a guide to His purpose for humanity. Until Christ returns, we need to study the prophecies to understand where we are and what God is doing.
Then Peter sounds his warning note: Do not presume to believe that your particular understanding of prophecy is THE correct one! He says this is the "first" rule of studying prophecy; it is something we must arm ourselves with at the outset. We must be humble enough to realize that our interpretation of prophecy is probably WRONG!
God's thoughts are far higher than ours (Isaiah 55:8-9); He does not think as humans do. Though we are surely growing in forming His mind in us (I Corinthians 2:16; Ephesians 4:13, 15; Philippians 2:5; II Peter 3:18), we still have a very long way to go! Paul puts it another way: "For now we see in a mirror, dimly, but then [in the resurrection] face to face. Now I know in part, but then I shall know just as I also am known" (I Corinthians 13:12). Or, as he quotes Isaiah in I Corinthians 2:9, "Eye has not seen, nor ear heard, nor have entered into the heart of man the things which God has prepared for those who love Him."
We, in this fleshly form, with our limited minds and perspectives, just do not know it all yet!