Active voice refers to a style of writing in which the subject of the sentence is the acting party. In passive voice, the subject of the sentence is acted upon, rather than performing the action.
Examples:
Flying through the air, the ball hit the girl. (Active -- "Ball" is both subject and actor.)
The girl was hit by the ball. (Passive -- "girl" is the subject, but "ball" did the action.)
I broke the vase. (Active -- I am the subject, and I did the action -- I broke the vase.)
The vase was broken. (Passive -- Getting broken happened to the vase.)
In general, readers prefer the active voice because clear, concise sentences hold a reader's interest.