One weak point can stop the service
An online shop may have several website servers but only one database. If that database fails, customers cannot check out. It is a single point of failure (SPOF) because no other database can take over.
The only internet connection or payment provider could also be a SPOF.
Ways to reduce the risk
Teams can add an independent spare database, offer limited service during an outage, or accept the risk if a spare costs more than a likely outage. Two servers that rely on one database may both be unable to handle orders when it fails.