What is Route reflectors? Why are these used?

In BGP there is rule called Split Horizon Rule as per the rule that any route received from an iBGP neighbor must not be advertised to any other iBGP neighbor.

This loop prevention mechanism induces a requirement that all iBGP routers in the network should be connected in logical full mesh topology to allow for routing information propagation through out the network. However, full mesh connectivity has never been a scalable solution specially in large networks.Two mechanisms were introduced to solve the scalability problem,by eliminating the need for the full mesh and in the same time keep the network loop free
1) BGP route reflectors 2) BGP confederations

BGP route reflectors
A route reflector is BGP router that is allowed to break the iBGP loop avoidance rule. Route reflectors can advertise updates received from an iBGP peer to another iBGP peer under specific conditions.
By breaking the rules, route reflectors are used to eliminate the full mesh requirement and allow for building iBGP networks that scale easily and cleanly.