If you search "pacing" in the dictionary, you will find that pacing is simply the rate of speed at which one walks. When talking about voice and speech, you can translate this concept into the rate at which one speaks.

One simple way of keeping an audience's interest is to vary your speaking speed. Compare the following (spaces indicate pauses):

Bad management costs jobs.
Bad    management     costs     jobs.

This is probably the single most important thing I've said.
This     is probably     the single    most important   thing    I've said.

In general, you should slow down to make your most important points. This gives your message time to sink in.