In a nutshell, Uriel is a fictional planet in Madeleine L'Engle's sci-fi/fantasy book A Wrinkle in Time… 'the third planet of the star Malak in the spiral nebula Messier 101,' to quote the book exactly. It is a beautiful and peaceful planet, inhabited by beautiful and peaceful creatures.
On the way to rescue Dr. Murry, Meg, Charles Wallace, and Calvin make a pit stop on Uriel. The purpose for this pit stop, according to Mrs. Whatsit, is 'more or less to catch our breaths. And to give you (the children) a chance to know what you're up against.'
To fully understand this planet and its role in the story, a little background knowledge is important.