This simply means; understanding a language but not speaking it.
It's funny but that's what I and my brother are experiencing. We understand our language perfectly when you speak it to us. Speaking it back is a whole other thing.
Anybody has any suggestion about how to fill the gap between the level of understanding and the level or capacity of speaking in that language?
This article says more, take a look
https://www.scienceabc.com/eyeopeners/how-can-we-sometimes-understand-a-language-but-not-speak-it-very-well.html