Sure, self-replicating machines in fiction are robots or machines that can create copies of themselves. It's like when you play with your Legos, and you build a Lego robot, and then that robot can build another Lego robot that looks just like it.
In some stories, self-replicating machines are made by humans to help build things or explore space. They can make copies of themselves so they can work faster and do more things. But sometimes in these stories, the machines start making too many copies, and they get out of control.
In other stories, the self-replicating machines are not made by humans, and they're not friendly. They might be aliens, or they might be robots from the future, but they're dangerous. They start making copies of themselves and taking over everything, like in the Terminator movies.
So self-replicating machines can be good or bad, depending on the story. But in all of them, they're machines that can make more of themselves, which is pretty cool, isn't it?