This an old saying, and a good rule to be aware of. It is doubly true when it is a little bit of knowledge of a foreign language.
Today I got myself into a bind by thinking I know Spanish better than I do. I won't explain the whole thing because it's too long and involved (and mostly because to do so would be too embarrassing). Just trust me when I say that you should never say 'Si' unless you understand all of what has just been said to you.
You'd think I would have figured that out many years ago, wouldn't you?
I'm lucky if I catch about every third word, because I'm trying to listen and translate simultaneously.