Summary: The prefix *pro-* primarily means "forward" but can also mean "for." Some words that the prefix *pro-* gave rise to are [*pro*]mise, *pro*, and [*pro*]mote. When you, for instance, make [*pro*]gress, you are stepping "forward," whereas if you give the [*pro*]s in an argument, you are speaking "for" something by stating its advantages.
