Ray Pritchard’s answer
I think it means to pray according to the leading of the Holy Spirit. When we pray, we can either seek to have our own will done or we can seek to have God’s will done. It means that we depend on the Holy Spirit to help us as we pray. It refers not to the words we say but the attitude of the heart as we pray. See When God Prays for You
and What is Praying in the Spirit?