Cofiwch Dryweryn means "Remember Tryweryn" in Welsh. Back in the 1960s, the government in Wales decided to build a dam in the Tryweryn valley, which is a place where people live and have their homes. The people who lived in Tryweryn were upset about this decision because it meant they would have to leave their homes and find new places to live.
Despite protests from the people in Tryweryn, the government went ahead with the dam and the people had to move away. This left the place they used to call home underwater, and they could never go back.
"Cofiwch Dryweryn" is a way for people in Wales to remember what happened to the people of Tryweryn. It's a phrase that reminds us to never forget their story and to always stand up for what we believe is right. It's important to remember the past so that we can learn from it and make sure that similar things don't happen in the future.