Chalk is a special word used in the military to talk about groups of soldiers who are all riding together in the same plane, helicopter, or other flying thingy.
Imagine you and your friends all getting on a really big yellow school bus to go on a field trip. You all sit together on the bus and your teacher tells you which bus number you are in (like Bus Number 1, Bus Number 2, etc.) so you know where to go.
In the military, the soldiers all get assigned to a chalk number instead of a bus number. So if you are part of Chalk 1, that means you and your fellow soldiers will all ride together in the same plane or helicopter for your mission.
It's important for the soldiers to know which chalk they are in so they can stick together and work as a team.