Post-exertional malaise is when your body feels very tired and achy after you do something physical, like playing outside or running around. It's like your body is saying "Hey, I need a break!" because it's working really hard.
This feeling can happen to anyone, but sometimes people who have a condition called chronic fatigue syndrome (also known as myalgic encephalomyelitis) might feel it more often and more strongly. Their body gets extra tired and achy and might need even longer breaks to feel better.
So, post-exertional malaise is like your body telling you it needs to rest after you do something tiring, but some people feel it more often and more strongly because of a condition called chronic fatigue syndrome.