Also called: downstream standby, tier-2 standby
In plain English
A cascaded standby is a replica that streams its WAL from another standby rather than from the primary. Its primary_conninfo points at an upstream standby (which must have hot_standby = on and spare WAL sender slots), and it is read-only like any standby.
Why it matters
Cascaded standbys are how you build replication tiers, but they carry one hard limitation: they can never be synchronous standbys, because the primary only tracks and waits on its directly connected children. Plan synchronous replicas as direct children of the primary.