In general terms a passively managed ETF or investment fund will follow a strict set of rules—such as tracking an index—which means it does not require active portfolio changes and just imitates the index holdings.. These strategies can capture a segment of the market in a cost-effective way but cannot act quickly to capture specific opportunities as they arise.
In an actively managed ETF portfolio managers make decisions related to the ETF on a daily basis. These can still come with rules, but the managers have more flexibility to capture opportunities as they see fit.
In the case of covered call option ETFs, a passive strategy would write call options at the same level each month. An active covered call option strategy allows portfolio managers to change the level of options the ETF sells.
This means the ETF can potentially generate more income or capture more upside opportunity based on the changing price of call options.