The Active Voice of a verb is the form it takes when the subject is actively performing it, and the object (if there is one) is the recipient of the action in a direct sense. For example:

The cat spat a rat on the mat. Active voice, with 'the cat' as the subject and 'a rat' as the direct object.
The cat sat on the mat. Active voice, with 'the cat' as the subject and no direct object.

Oppose passive voice and compare middle voice.