As we all know, language can’t be translated word by word. Everyone that has tried to learn a new language has certainly fallen into the trap of looking at individual words and translating them back into our own. The results can be extremely embarassing/funny/awkward/confusing.
In American English, when we say that we know something like the back of our hand, this is saying that we know something very well. You could say that it’s something that we know by memory — something we don’t have to think about.
“I don’t need you to write the lyrics to that song, I know them like the back of my hand.”
“I’ve read that book so many times, I know it like the back of my hand.”
“I’ve driven there so much that I don’t need to use my GPS. I know the route like the back of my hand.”