If we are to have consistent answers to prayer, the words of Jesus must abide in us. That is, as we saw from verses 4 and 5, Jesus himself must abide in us speaking. We do not just stock ourselves with dead ideas which he spoke once, but we receive and believe and remember and meditate on the truths that he spoke once and is speaking now as he abides in us. If the words of Jesus abide in us, then the result will be that you pray with power and effectiveness. There are four truths about prayer that come from meditating on this verse in its wider context in the writings of John.
Prayer is for fruit-bearing.
God designed prayer to give his disciples the joy of bearing fruit while God himself gets the glory. We can see this in the connection between verses 7 and 8 and then in verse 16. If you abide in me and my words abide in you, ask whatever you wish, and it shall be done for you. He gets glory as the one who is rich and good enough to answer prayer. But also . . .