Okay, so you know how when you say goodbye to someone you might wave or give them a hug? Well, a benediction is like saying goodbye to a group of people, but instead of waving or hugging, you say a special prayer or blessing for them.
It's like when your parents tuck you in at night and say "I love you" or "sweet dreams", a benediction is a special way for someone (like a priest or pastor) to send you off with good wishes.
During a benediction, the person leading the prayer might ask for blessings on each person in the group, or they might ask for peace and goodwill in the world. It's a way of saying "see you later" and letting everyone know that they are loved and appreciated.