This is a harder concept to explain to young kids. Some days they are also just going to be tired. If a sibling is sick and wakes up crying throughout the night. If we take a very early or a very late flight for a vacation. If they attend an event that goes past their normal bedtime. Or if they push themselves extra hard during a class or a camp.
When children misbehave, it's so easy to quickly say "oh, they are just tired" and excuse the bad behavior. But at a certain age, I think it's important for kids to learn some strategies to cope when they are tired. Will they nap? Spend more time reading/coloring and less time running? Sleep in late? Go to bed early for a few days? Eat more nutritious food?
Whenever our daughter is overtired, my son and I work together to figure out ways to help her. Maybe she should take an early nap. Maybe I should wear her in a carrier. Maybe we should read extra books. Maybe she should go to bed early. Sometimes our son comes up with the perfect way to calm her down. One day, maybe she can return the favor and help him when he is overtired.