An autonomous system (AS) is a collection of connected Internet Protocol (IP) routing prefixes managed by one or more network operators on behalf of a single administrative entity or domain. It has a single and clearly defined external routing policy.
An autonomous system number (ASN) is a unique number used to identify an autonomous system. It enables that system to exchange exterior routing information with other neighbouring autonomous systems.
A detailed two-part blog post which explains the fundamentals of these systems and the role of AS is available
The AS Info APIs provide valuable information about the autonomous systems which allows you to identify the ownership of the systems and discover their neighbouring systems.