The Bowl is a round bowl-shaped room filled with starving electric rats.
We see the Bowl in Book 2 of the Michael Vey series: Rise of the Elgen. It does exactly as the description says, keeps the electric rats fed with live food and contains them inside. It's mostly like a large cell, for thousands and thousands of tiny rodents.
They send live food such as bulls and humans down the shaft into the bowl for the rats to eat.