Justin Wilson has a great article on How Route Servers Work on an Internet Exchange Point (IXP) on his blog.
A route server is a specialized software system or device at an IXP. Its primary purpose is to simplify routing information management between participating networks, also called autonomous systems (ASes). Instead of each AS having to establish individual bilateral BGP (Border Gateway Protocol) sessions with every other network at the exchange, the route server allows participants to set up a single connection.
This “one-to-many” relationship significantly reduces administrative overhead and enhances scalability at the IXP.