If you say something off the top of your head it means that you give an answer to a question from memory or based on the information that you have. It is not necessarily 100% accurate. It is a guess or an estimation.
This is a common expression and it will heal you sound more like a native English speaker.
I don't know how much a pizza costs in London but off the top of my head I would say 10 pounds. That price sounds about right.
I don't know what Geoff's address is off the top of my head. I'll have to look it up! Can you wait a minute.
Do you know how old I am? No. But off the top of my head I'd say 45. Am i correct?
Now try and incorporate the expression into your vocabulary.
Here's another English expression for you to learn.
Please take a look at my English lessons on curious.com