If a verb is in the Active Voice then it shows the subject performing an action:
the subject does something
If, however, a verb is in the Passive Voice then it is the other way around and the subject receives the action:
something happens to the subject
For example look at these two sentences:
She broke the window.
The window was broken.
In the first example the subject - she - does something: break the window. In the second example the subject - the window - has something happen to it.
Generally you should try to use active tenses over passive tenses. This is because active tenses are simpler to understand. Passive sentences are used more, for example, in scientific journals and in situations where you do not want the subject to be obviously known.
For example, using the passive we can say:
A man was killed in town today.
By doing this we don't know who killed the man. Perhaps it was a terrorist or perhaps it was a policeman. Depending on your point of view, you may or may not want to advertise who killed the man.
On the other hand, using the active voice leaves us in no doubt:
A bolt of lightning killed a man in town today.