Greek nationalism is a feeling of being very proud to be Greek and believing that Greece is the best country in the world.
Just like how you might wear a shirt with your favorite superhero on it and feel really happy and proud, Greeks feel proud and happy to be Greek. They might show their Greek pride by wearing a t-shirt with the Greek flag on it or cheering really loudly when their favorite Greek soccer team wins a game.
Sometimes, Greeks might also want to make their country even better. This is where the idea of Greek nationalism comes in. They might think that Greeks should be in charge of their own country and making their own decisions. This idea can come from wanting to have more control over what happens in Greece, wanting to protect Greek culture and traditions, or wanting to make Greece stronger and more independent.
Overall, Greek nationalism is about feeling really proud of being Greek and wanting Greece to be the best it can be.