Christians have different opinions on what it means to pray in the spirit. Some people believe that it refers to a part of St. Paul's writings that say that the Holy Spirit prays on people's behalf when they don't know how to pray. Others believe it refers to glossolalia.
Glossolalia is also called speaking in tongues. It is the practice of vocally producing sounds and syllables that lack any comprehended meaning. Some people believe that these vocalizations represent a type of spiritual language. Many Pentecostal and Charismatic Christians believe that praying in the spirit means to speak in tongues while engaging in personal prayer.