01Plain English
The simple explanation
An ASBR introduces a route from outside the OSPF routing domain.
02Under the hood
The technical explanation
AS-external LSAs describe external prefixes, metrics and forwarding information. They flood through the OSPF domain except areas that exclude them, such as stub areas and NSSAs. E1 adds internal path cost to the external metric; E2 primarily compares external metric, using internal cost for relevant tie-breaking.
Protocol reference: RFC 2328 (OSPFv2). NSSA reference: RFC 3101.
logic
Make it stick
The friend with news from a completely different group chat.
03On the wire
Example
A controlled redistribution policy can introduce a static prefix as external OSPF information. Never redistribute everything merely to make one route appear.