A single point of failure (SPOF) is any component whose failure brings down the system. One database server. One load balancer. One data center.
Find SPOFs by asking: "If this dies, what happens?" If the answer is "everything breaks," add redundancy.
In interviews, interviewers look for SPOF awareness. Point them out proactively. "This database is a SPOF. I'll add a replica for failover."