What does 1 Corinthians 11:21 mean?

This is part of a running rebuke of the Christians in Corinth for their wild misuse of the Lord's Supper. The previous verse indicated that what's happening is so contrary to God's intent that it doesn't even make sense to call it the Lord's Supper! Paul's criticism is that these believers are misunderstanding the meaning of that sacrament. They are not taking part in it in order to remember Christ's sacrifice of blood and body to pay for their sins on the cross.

Here, Paul describes what has been reported to him. The early church practiced the celebration of the Lord's Supper much differently than most churches do today. That, itself, is not a problem. Scripture gives extremely wide latitude in how communion can be practiced. In most cases, in the very early church, the Lord's Supper involved eating an actual meal together. This was fine when handled with respect and honor towards the Lord and all those in attendance.

Clearly, that's not what the Corinthians were doing.

Based on what Paul says here, it's likely each person brought their own separate stock of food. In order to accommodate many people, without involvement in a temple, they likely gathered at the home of a wealthy member of the congregation. Those who were wealthy could bring plenty of food for themselves. The rich could afford to indulge in abundant alcohol. Those living in poverty, however, had little to bring and eat.

The result of that disparity was a "communion" where some participants went hungry, merely watching the wealthier believers eat and drink. In addition, drunkenness was at least common enough for Paul to mention directly. Paul's exclamation of shock in the next verse seems fitting.