Welsh nationalism is an idea in Wales that people who live there should have more control over their own country. It's kind of like wanting more say in the games you and your friends play, or wanting a bigger slice of cake at a birthday party.
The people who support Welsh nationalism believe that Wales should have its own government and laws, separate from the government and laws of the bigger country it's a part of, which is called the United Kingdom. This would be like having your own special club for just you and your closest friends, where you all make your own rules and nobody else can tell you what to do.
People who support Welsh nationalism also often believe that the Welsh language and culture are really important, and they want to keep them alive and thriving. This would be like really loving your favorite toy or stuffed animal and wanting to take really good care of it so it lasts a long time.
So, Welsh nationalism is all about wanting more control and autonomy for Wales, and preserving the unique Welsh language and culture. It's kind of like wanting to make sure your voice is heard, no matter how small you might feel sometimes.