Okay kiddo, have you ever done something naughty and then your parents forgave you and let you off the hook? That's kind of what clemency is!
Clemency is when someone in a position of power, like a president or governor, decides to forgive someone for a crime they committed or shorten their punishment. It's like a big person saying "okay, I know you did something wrong, but I'm going to make it easier for you."
Why would someone do this? Sometimes, the person who did the thing wrong is really sorry and promises they won't do it again. Other times, people think the punishment was too harsh and they want to help the person out. For example, if someone got in trouble for something small, but they got a really big punishment, someone might use clemency to make it easier on them.
Basically, clemency is like a second chance or a way to make a punishment less severe, given by someone who has the power to do so.