What does Matthew 10:9 mean?

Jesus is instructing the twelve apostles about how to carry out the mission He is giving to them. They are being sent to preach the gospel of Jesus and the kingdom of heaven. To do so, they will demonstrate its power with miracles of healing and casting out demons (Matthew 10:5–9).

The apostles have been ordered not to charge any money for this good news. In this case, this goes beyond the idea that the gospel, itself, is not "for sale." This is a specific mission with a specific purpose, and to accomplish that purpose, the apostles must demonstrate a radical dependence on God (Matthew 6:25–34). For this task, they are not to acquire gold, silver, or copper. These either represented money or metals that could be traded for cash, goods, or services.

In short, they are to take no material preparations of any kind; their needs will be met by God's provision through the people they serve.

Taken in context with Jesus' other words, and the teachings of the New Testament, it's clear this command applies only to this mission (Luke 22:35–41). Jesus' does not intend us to believe neither the disciples, nor modern believers, should ever gather resources for another mission. It does not mean missionaries or church leaders should rely on those they serve to provide all their needs. Jesus wanted these twelve men to learn to rely on God for what was needed on this missional outing.