The business requirements document should accurately, and in detail, describe the purpose of the project. It states what is needed to achieve that goal i.e. what is in-scope, what is out-of-scope, any assumptions that have been made, any constraints that have been imposed and expected timescales.
The role of a project manager is very diverse and one of the most vital skills is good people management. The most carefully planned projects will go awry if the team is not motivated, encouraged and kept informed.