What does John 14:2 mean?

This verse uses the term monai, which most literally means "dwelling places." Translations such as the KJV have rendered this as "mansions." Many have latched onto this in a very literal way, imagining that Jesus is promising physical palaces for all Christians in heaven. While that's not entirely impossible, there's a more important meaning here. Jesus says these monai are in His "Father's House," using the expression tē oikia, which can mean a physical house or a family. In this context, it seems to mean something more family-related. Christ's meaning here is more likely a reassurance that in the family of God is room for all of them, more so than a promise for a fancy house. That's more in keeping with Jesus' later comment in this verse that He's preparing "a place" using a very generic Greek term, topon.

Jesus' remark here is meant to continue the reassurance He began in the prior verse. Like that verse, this one contains an expression which can be translated in more than one way. Translators have rendered this phrase as a rhetorical question, a direct statement, and either included or separated the comment about a "place" into the sentence. Which of those was John's original intent is an answer deeply buried in ancient Greek grammar and vocabulary. No matter which is ultimately the case, the practical meaning is the same: Christ has not been deceptive, there is restoration at the end of a believer's life journey, and this destiny is the result of Christ's efforts.

Jesus will use the same word for "dwelling places" in John 14:23 when He speaks about coming to make His home in those who believe.