"Likewise the Spirit also helps in our weaknesses. For we do not know what we should pray for as we ought, but the Spirit Himself makes intercession for us with groanings which cannot be uttered. Now He who searches the hearts knows what the mind of the Spirit is, because He makes intercession for the saints according to the will of God." -Romans 8:26-27 (NKJV)
Additional Scripture
-Jude 20-21 (NIV)
-1 Corinthians 14:2 (NIV)
Most of life is spiritual, comprising things we can't grasp with our physical senses and mysteries we can't fathom with natural thinking. Good intentions and masterful plans don't really make much progress against spiritual forces. That's why we pray.
But when we don't know what God wants to do in our situation, or when we can't find words to adequately express what's in our heart, how do we pray? Thankfully, we're not left on our own to figure out what to pray. The Holy Spirit offers to lead us.
The Spirit knows what's in God's heart and exactly what He wants in our life. He directs our intercession accordingly, leading us to pray about spiritual mysteries and God's will for our life.
The Spirit may also put words on our lips to express sentiments that are beyond our understanding. Because God's thoughts aren't like ours, Spirit-led prayer words don't make sense to our minds. Our "mother tongue" enables prayer for what we do understand, and our "other" tongue allows us to pray about what we don't.