This is a public proclamation from Allah and His Messenger to the people on the day of the Great Hajj (Pilgrimage) that Allah and His Messenger do hereby dissolve treaty obligations with the disbelievers. Therefore, if you repent, it will be better for you but if you turn away, then you should know that you cannot frustrate the Will of Allah. O Prophet, proclaim a painful punishment to those who are unbelievers.
Meaning:-
Proclaim: declare officially, announce officially or publicly.
Treaty: any agreement, contract, covenant.