Imagine you have a picture of a dog, but instead of the word "dog" written underneath it, there's a word that says "cat." This is what we call a "form-meaning mismatch." The form (picture) and the meaning (word) don't match up correctly.
In other words, when we use words, we are trying to communicate a specific meaning to others. If the words we use don't match up with the thing we're describing, it can be very confusing for the person we're trying to talk to.
For example, imagine that you have a toy truck that's green, but you accidentally call it "blue." Your friend might get confused and not understand what you mean when you say "green truck" later on. Even though you know you meant "green," because the words you used didn't match up with the actual thing you were describing, there was a form-meaning mismatch.
Form-meaning mismatches can happen in languages other than English, too. Sometimes, people use the wrong word or phrase to describe something, which can make it hard for others to understand what they're saying. But don't worry - everyone makes mistakes and it's all part of learning and growing!