There are 3 kinds of tongues: 1. Tongues of man: these are the lowest level of tongues, 2. Tongues of angels: these stir up angelic activity, and everytime you pray in tongues, angels of God start moving on your behalf, and 3. Unknown tongues: these are the highest level of tongues. The devil and man do not understand these tongues because only God can. The Holy Spirit makes mysteries known to what man and the devil don't know through the language of unknown tongues. There's a lot of things we don't know. When you pray in the spirit, in unknown tongues, you have access to know things in the spirit, you can know something you didn't know before. The moment we receive the Holy Spirit God authorizes us to receive and know His mysteries.