In many sentences, the subject performs the action of the verb:

The girl ate the cake.

This is called active voice. In passive voice, the subject receives the action of the verb. In other words, the subject is acted upon:

The cake was eaten by the girl.

Sometimes passive voice makes for an awkward-sounding sentence:

Dinner will be cooked by Mary after Sunday's meeting.

Other times it can create confusion or lack precision:

A new file system was put into place.

But who put it into place?

Generally, active voice is smoother, more precise and concise. (It usually takes less words: was eaten vs. ate, will be cooked vs. cooked.) These are great reasons to use it. But don't abandon passive voice entirely. You can use it to create emphasis on the result or receiver of the action.

The gun, gray and cold, was held to her temple.

This dramatically keeps the focus on the gun.

Here's another example:

The butlers arrived the next morning. Books were put back on the shelves. Shards of glass were swept away.

The transformation of the house here is more important than the activity of the butlers.

Don't get in the habit of using passive voice regularly, but some well-chosen moments of this sentence construction can add just the right touch to a moment.