A capper is something that is put on top of something else to finish it off or make it complete.
A capper can make people think of finishing a job or making something perfect.
Examples:
The capper goes on top of the bottle.
I need a capper to finish the project.
The capper makes the whole thing look better.
The capper is the best part!
The capper makes everything perfect.
The capper is the last thing you need to do.