Ambiguous grammar is like when someone gives you a sentence to read, but you're not sure what it really means because the words can be understood in different ways.
Let's pretend your mom says, "I saw the man with the telescope." Does that mean your mom had a telescope and saw a man? Or does it mean she saw a man who had a telescope?
This is an example of ambiguous grammar because the sentence can have two different meanings. And you might not know which one is true unless your mom explains it more clearly.
Ambiguous grammar happens when the words in a sentence can be put together in different ways, and it's not clear which way is the correct way to understand it. This can make it hard to communicate clearly, and it's important to try to use clear language so people can understand what we mean.