Check out the example folder to see how the pattern is used.
Real-world example
Take the example of a code / document editor (i.e. originator), where whenever you write something, the last content is saved in memory (i.e. memento) so that you can get back to it and maybe get it restored using some action buttons or keyboard shortcuts (i.e. caretaker).
In plain words
Memento pattern is about capturing and storing the current state of an object in a manner that it can be restored later on in a smooth manner.
Wikipedia says
The memento pattern is a software design pattern that provides the ability to restore an object to its previous state (undo via rollback).