To "pass out" means to lose consciousness or go to sleep in a kind of messy and sudden way. Here are some situations in which a person might pass out:

• after drinking a lot of alcohol
• when standing outside under the hot sun without having enough water to drink

So you can see that "passing out" is a kind of medical emergency. But you can also use this phrase to talk about going to sleep when you're very tired, like after working hard all day:

He passed out as soon as he got home, and he's still asleep.

When you use "pass out" in this way, it still sounds sudden but not quite as dangerous as "passing out" because of a health problem.