ASN

An Autonomous System number that participates in route exchange between networks on the internet.

In plain words

ASN is a network's unique identifier on the internet. You can use it to find which provider or company owns an IP.

Practical example

AS15169 belongs to Google; seeing this ASN in GeoIP tells you the IP belongs to Google.

Where you encounter it

When analyzing who owns an IP address and which provider carries the traffic.