This summary is not the rule. The rule text controls.
If someone brings up part of a document or recording during a trial, the other side has the right to ask that the rest of it be shown too, right then and there. This stops people from using a piece of a statement in a way that makes it misleading or unfair.
For example, if a lawyer reads one sentence from a letter that makes their case look good, the other side can ask that the full letter, or at least the parts needed to give a fair picture, be brought in at the same time. This way, the judge and jury get the complete context instead of just the part that helps one side.
The goal is fairness. Words taken out of context can create a false impression, so this rule makes sure the whole story is available when someone starts using a document or recording as evidence.