A simple analogy

Think of a jury in a court trial.

A jury cannot deliver a verdict unless a minimum number of jurors agree. That minimum number is the quorum.

Distributed systems work the same way. Before an operation (like a read or a write) is considered successful, a minimum number of servers must agree on it.

The definition

A quorum is the minimum number of servers that must successfully perform an operation before we declare the operation successful.

A simple example

Suppose our database is replicated across 5 machines.