@hyuna27g_ The verb 'to pen' comes from farming. A pen is where animals, like sheep, are kept, so to pen the sheep in, is to get the sheep into the shelter, so they can't wander free across the hills. In football to pen a side in, means to keep them confined to the defensive third of the pitch. The team that is penned in cannot 'get out' of their half for example.

 

Yes, if a team is penned in it means they cannot escape from their own penalty area/half. 

Here's an example from the BBC website (2020)

They did create chances on the break despite being penned in their own half for the majority of the game