Ludonarrative dissonance means that the story of a game and the way you actually play the game do not match up or fit together well. This can happen when the story of a game tells you to be a good person and do the right thing, but the game encourages you to do bad things to succeed.
For example, let's say you are playing a game where you are a superhero who is trying to save people from bad guys. But in the game, you can also choose to hurt innocent people or steal things from them to get ahead. This doesn't match up with the idea of being a hero and doing good things.
So basically, ludonarrative dissonance is when the story of a game doesn't match up with the gameplay itself. It can be confusing and make it hard to understand what the game is trying to say or teach us.