I see two possibilities of interpretation for that scripture.
One can be simply praying in Gods will, taking guidance and leading from the Holy Spirit.
The second can be praying in spiritual tongues, which is utterance by the Holy Spirit. This is when we do not know what to pray for and the Holy Spirit prays for us in intercession.
Rom 8:26 Likewise the Spirit also helps our infirmities. 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. Rom 8:27 And He searching the hearts knows what is the mind of the Spirit, because He makes intercession for the saints according to the will of God.