I haven't watched that episode, and normally wouldn't take a studio prop and the overdramatization seriously. But what it means can be broken down easily:
Cabin altitude is a measure of the cabin's pressurization. 10,000 feet means the pressure inside the cabin is that of 10,000 feet outside the plane. The higher this value, the lower the pressure.
Trapping the outflow valve means the outflow valve closes. The outflow valve opens and closes to regulate the cabin pressure.
When it is fully closed, the cabin pressure won't decrease unless there's structural damage causing the pressure to escape. The valve closes if the cabin altitude reaches 10,000 feet.