To have the wrong idea about something. For example if one of your friends thinks a girl likes him but you know she doesn't, or you think he has no chance, you might say he's barking up the wrong tree.

"Barking up the wrong tree" is an idiomatic expression in English, which is used to suggest a mistaken emphasis in a specific context. The phrase is an allusion to the mistake made by dogs when they believe they have chased a prey up a tree, but the game may have escaped by leaping from one tree to another. The phrase means to mistake one's object, or to pursue the wrong course to obtain it. In other words, "if you are barking up the wrong tree, it means that you have completely misunderstood something or are totally wrong."

The question was for those actually trying to learn English on this site.

Obviously. What is less obvious is why you are asking the question...

If you are native English speakers, you obviously know the answer.