An Autonomous System Number (ASN) is a globally unique number assigned to an autonomous system — a collection of IP prefixes under a single administrative domain with a common routing policy. ASNs are used by internet service providers, enterprises, and hobbyists to participate in Border Gateway Protocol (BGP) routing on the internet.
With your own ASN you can announce your IP address space to the internet from any location or provider, giving you true provider-independence and multi-homing capability.