What does Matthew 6:7 mean?

It's not just Christians who pray. Most religions include something akin to prayer, whether that means communicating with a deity, spirits, ancestors, or angels. Others involve meditations or chants which are inwardly focused. Most Gentiles in the first century participated in pagan idol worship; this involved repetitive chanting of words and phrases. Some thought they would be heard and receive their requests for repeating their prayer an excessive number of times in a row.

It's possible some Jewish people in Jesus' era had taken on those superstitions and prayed in repetitive, mechanical ways. Even today, there is temptation to simply repeat words and call it "prayer." Or, to insist on using only certain phrases, languages, or approved expressions when communicating with God. This does not mean all repeated words or pre-written prayers are wrong. It means that words, in and of themselves, are not the point of prayer (Romans 8:26). If we're not sincerely communicating with God, from our hearts, then we're not praying, in a godly sense. This is in keeping with Jesus' teaching that motives matter as much as actions (Matthew 5:20; 6:1).

The Lord's Prayer, which Jesus will soon present as a model (Matthew 6:9–13), is often misused, in an example of what Jesus is warning about. Praying those exact words is not wrong—at all—but that arrangement has no special power. Prayer is not a magical incantation. The words we pray should be expressions of our hearts, not mechanical echoes.

Jesus will specifically point out (Matthew 6:8) that God doesn't give points for mindless repetition. He doesn't need to hear our words repeated over and over to understand the message. He gets it the first time.