This summary is not the rule. The rule text controls.
If you agree to a judgment against you in the trial court, or if you go along with it willingly and without any conditions, you give up your right to appeal that judgment. Basically, once you accept the outcome on your own terms, the law treats that as final and you can't turn around and challenge it later.
There's an exception for judgments that have multiple separate parts. If you accept or agree to just one part of a judgment that can be divided into pieces, you can still appeal the other parts. The same goes for judgments that can't easily be split up: if you accept the part that works in your favor, you can still challenge the other parts that don't.
In short, this rule protects the fairness of the appeals process. It stops people from accepting a result and then trying to appeal it anyway, while still allowing them to fight the parts of a decision they never agreed to.