It's often used to describe games or entertainment that are meant to be fun and relaxing, rather than challenging or educational. The website Friv.com, for example, is known for hosting a wide variety of casual games, ranging from simple puzzles to arcade-style action games.
So, when you see the word "Friv," think of something fun and lighthearted, like a silly joke or a game you play just for the enjoyment of it.