In computer networking, peering is a voluntary interconnection of administratively separate Internet networks for the purpose of exchanging traffic between the customers of each network. The pure definition of peering is settlement-free or "sender keeps all," meaning that neither party pays the other for the exchanged traffic; instead, each derives revenue from its own customers.
Peering requires physical interconnection of the networks, an exchange of routing information through the Border Gateway Protocol (BGP) routing protocol and is often accompanied by peering agreements of varying formality, from "handshake" to thick contracts.
Marketing and commercial pressures have led to the word “peering” being also used routinely when there is some settlement involved, even though that does not correspond to the original technical meaning of the word. The phrase "settlement-free peering" is in turn used to unambiguously describe the pure cost-free peering situation.
In layman terms, peering means sharing data with other users of your ISP at blazing fast speeds (100mbps). This won't cost your ISP anything because the data is travelling only through their network and not outside. Fivenet has this if i remember correctly.