So, Welsh triads are like lists of three things. It's sort of like when you say "three blind mice" or "three little pigs." In Wales, they like to make these lists of three things that are important in their history or culture.
These lists are usually about heroes, places, and important events. They help people remember important things and they are really fun to say because they sound like riddles.
For example, one Welsh triad is "Three things that are wise: to love, to be patient, and to keep learning." Another one is "Three things that make a person happy: a loving family, good health, and a peaceful mind."
People in Wales have been making these triads for a long time and they still do it today. It's a way to keep their traditions alive and remember what's important to them.