Paul Revere was one of many riders who left the environs of Boston to warn the country side of the impend British march on Concord and Lexington. His route was completed by another rider, Dr Prescott, after Paul Revere was captured by a British patrol.
According to Wikipedia, Revere told his capturers about the British movement from Boston and about the probability of a fight near Lexington as that is where the militia was assembling.
You have to remember that the Longfellow poem, which most, if not all, history books are based was written in 1851, 76 years after Paul Revere’s ride (1775) and 40 years after his death.