What does the "Reset Maximum HP changes during this rest" checkbox do?

Not sure what this checkbox in the Short Rest panel does.

Thanks in advance!

If a creature has reduced your max hp, and you've adjusted that on your character, sheet, ticking this checkbox will undo that on a long rest.

Thank you. To make sure I understand, you said:

"If a creature has reduced your max hp, and you've adjusted that on your character,..."

Under what circumstances would this occur? Is this like an Energy Drain or something? Sorry, I'm still pretty new to 5e!

Thanks!

There are some creatures with effects that reduce your maximum hp, such as the wraith's Life Drain which says

The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken

Okay, got it. Thanks so much!