A protection agreement is an agreement that is customarily put in place when a carrier is withdrawing from or suspending a market they were previously selling tickets for. Basically it is an agreement for another airline to accept UA's passengers/receive payment to fly them to the suspended/withdrawn market under specific rebooking parameters (booking codes, routing, etc.) even though they did not originally participate in the fare. Absent a protection agreement, the only carriers UA would be able to these passengers on are ones who had originally decided to participate in the fare, which depending on the fare filing may leave few or no options available.

If a protection agreement is put in place, it wouldn't mean UA is reinstating service themselves (the seasonal suspension is happening, regardless), just that they have an option to rebook passengers who were previously ticketed on the IAH-SCL flights during the suspension timeframe..