By stropping, you remove any burrs that may still be present on the cutting edge after using a stone as well as polishing it at the same time.
Stropping is used with a variety of compounds each of a different consistency from course to super fine. You apply the compound to your strop depending on what you would like to do, and then strop your knife at an even angle on both sides until the wanted result is accomplished.
If you're a Scandi guy like myself that usuallly means you'll be playing with carbon steel, a good stropping is all you'll need most of the time to get back a razor like edge. That's why allot of guys here love Mora's, you can have a superior razor sharp edge back in no time after a few passes on a strop.
Try doing a search on "stropping" you'll see tons of threads posted on the subject here over the years.
Cheers,
Serge