This summary is not the rule. The rule text controls.
Hearsay is when someone repeats a statement made outside of court to prove that what the statement says is true. This kind of secondhand information generally cannot be used as evidence in a trial.
The reason for this rule is fairness. If a statement is used as evidence, the person who made it should be in court, under oath, where they can be questioned about it. Allowing hearsay would let people bring in claims without giving the other side a chance to challenge the person who actually said them.
There are exceptions to this rule. Other parts of the evidence rules, or other laws, may allow certain kinds of hearsay to be used in court. But unless one of those exceptions applies, hearsay stays out of the case.
