Interesting question. The word "off" here is a preposition. It means "from" as in "to be separated or removed from something else". This is the literal meaning. The literal interpretation is that the idea or explanation which you know very well, sits at the top of your head. If you don't know the concept very well, then it is not on top of you head so then you can't get it "off the top of your head".
This phrase is idiomatic and should not be interpreted literally. English speakers are definitely not thinking about any literal meanings when they hear or utter this phrase.
Therefore the most useful interpretation of this phrase is that you cannot provide the information sought without some time to think or other aids like a book or the internet, which ties in with the "not immediately" connotation as in the answer given by Altini.