What does it mean to have an open interest limit?

Brokers are subject to position limits in addition to the exchange-mandated initial margin requirements. The term "open interest" refers to the quantity of futures (or options) contracts that are now outstanding (open) in the market.

There is a client-by-client open interest limit of 5% of the total number of all derivative contracts for the same underlying, and a trading member-by-trading-member open interest restriction of 15%. (brokers)