The Internet is a network of networks, based on the Internet Protocol (IP). Internet Exchanges (IXs) have an efficiency role in the infrastructure. At an IX, the networks of Internet Service Providers (ISPs), telecommunications carriers, content providers, webhosters and the like, meet to exchange IP traffic with one another. This exchanging of regional, national and/or international IP traffic is generally known as "peering".
Every member or customer at an AMS-IX Internet exchange is in the position to peer with any of the other connected parties although they are not obliged to: each network might have a different peering policy, and this policy may differ depending on the party that is negotiating with them. Peering is mostly done without an exchange of money and based on a situation where parties see a mutual benefit.
This mutual benefit is generally established by the (type of) traffic and routes sent and received, so unless parties have an open peering policy, they will peer with parties of about the same size. Open peering policies make sense if you have a lot of content to distribute that is interesting traffic for access networks or if you are a party that whishes to offload as much traffic as possible by peering to decrease traffic that has to be sent via the commercial route (buying transit).
In general, parties peer at an IX to decrease network costs, to improve network performance and to make their network more redundant. Improving network performance is done by accessing many networks directly at the exchange that otherwise would have taken several network 'hops' through other parties. The redundancy is served by having many routes at the exchange through which traffic can be sent. Into play comes also that peering networks prefer the autonomy over their own network and traffic.