In active voice, the subject performs the action. Example: "She [ate] the ice-cream."
In passive voice, the subject receives the action. Example: "The ice-cream [was eaten] by her."
Use active voice when the doer of the action is more important or you want a clear and direct statement.
Use passive voice when the receiver of the action is more important, or you don't know or don't want to mention the doer of the action.
Mastering active and passive voices will enable you to convey your message effectively in different situations.