Mr is used before a man's name when you are speaking or referring to him.
Mr Grant.
格兰特先生。
Mr Bob Price.
鲍勃·普赖斯先生。
Mr is sometimes used in front of words such as 'president' and 'chairman' to address the man who holds the position mentioned.
Mr President, you're aware of the system.
总统先生,您是了解该体制的。