Yescymru is a group of people in Wales who want their country to become independent from the United Kingdom. This means they want Wales to have its own government and make its own decisions, like what kind of laws to have and how to spend its money.
The word "Yes" in Yescymru means that they are saying yes to independence for Wales. The "cymru" part is a Welsh word that means "Wales".
Basically, some people in Wales think it would be better for their country if it was independent, and Yescymru is a group that supports that idea. They want Wales to have control over its own future!