1. You must be...:
a grammar point used to draw a conclusion about a person based off of some evidence
- Your flight arrived late last night. You must be tired.
- You haven't eaten all day. You must be hungry.
2. to be the head of something:
to be the leader of something
- He is the head of marketing in our company.
- She is the head of this project.
3. to work closely together:
to work together on many things and with great cooperation
- We worked closely together on this project.
- We have worked closely together for 20 years. So, we know everything about each other.