What does Matthew 25:21 mean?

A parable about a master and three trusted servants communicates to Jesus' servants how they should live while waiting for His return (Matthew 25:14–20). The master in the story left town after giving these three servants large sums of tradable cash. These are counted using the measure of "talents," which in the ancient world was an enormous weight of precious metals. A single talent might have been as much as a common laborer could earn in twenty years. Now the master has returned after a long time away and has come to these servants to settle accounts with them. He wants to see what they have done with the money he has left them.

The first servant has made his report. He used his master's five talents to earn another five talents. The master trusted the servant with great wealth, and the servant earned the equivalent wealth for his master.

Now the master responds. He calls the man a good and faithful servant and tells him "well done." He rewards him in two ways. First, he tells the servant that because he has been faithful over little, he will make the servant responsible for a great deal more. Second, he welcomes the servant to share in his own personal joy. In other words, the master welcomes the servant into closer connection to himself, managing more of his own estate and walking in joyful relationship with him.

Jesus is showing that those who use what He gives to them to increase what is His will also be rewarded with greater opportunity and joy when He returns.