If something is pivotal, it is very important and everything else depends on it.
Pivotal things are like the most important part of a big machine. Without them, the machine doesn't work.
Examples:
The captain's role is pivotal to the team's success.
Her idea was pivotal to the project.
The pivotal moment in the story is when the hero saves the day.
The cake was good, but the ice cream was pivotal!
The pivotal key to the treasure chest was hidden under the bed.
The pivotal piece of the puzzle was missing, so we couldn't finish it.