Nested quotation is like playing a game of "telephone" where you pass a message from one person to another. Except, instead of just passing a message, you pass a quote.
Let's say you want to tell your friend something that someone else said. You would start by saying "My mom said..." and then you would repeat what your mom said.
But what if your mom was actually quoting someone else? Then you would have to use nested quotation. You would say "My mom told me that her friend Sally said..." and then repeat what Sally said.
So, basically, nested quotation means you have to repeat what someone else said within another quote. It's like a quote within a quote, and you have to keep track of who said what. It can be a little confusing, but it's important to make sure your message is clear and accurate.