Learn Swedish: Var och en checkar in och får en flytväst. - Each one checks in and gets a life jacket.

While alla (everyone or all) refers to the group as a whole, var och en (each one) emphasizes the individuals within that group. In this sentence, it highlights that every single person goes through the check-in process and gets a life jacket individually.

Why is it var och en and not vart och ett here?

You use var och en when referring to people, because nouns for people are almost always en-words. You would only use vart och ett if you were referring back to a specific group of ett-words, like hus (houses) or barn (children).

Is checkar in a particle verb, and how does that affect pronunciation?

Yes, checka in (to check in) is a particle verb. In Swedish, the spoken stress in a particle verb almost always falls on the particle, not the verb itself. When speaking this sentence, you should put the emphasis heavily on the word in.

How is the word flytväst built?

It is a compound word created by combining the root of the verb att flyta (to float) and the noun en väst (a vest). Swedish frequently builds compound nouns this way to describe exactly what an object is used for.