Explanation of playerWhoIsIt and TagPlayer

What is the meaning of "playerWhoIsIt" concept applied in the Marco Polo tutorial? I read at least 3 times and I don't get it. I searched through the forums and documentation but still can't reach a clue.

- What is the purpose of keeping playerWhoIsIt ID?
- What is the purpose of tagging a player?

Sorry if is a really dummy or newbie question but I don't want to miss anything from the beginning of learning about Photon.