Trolling is a fishing method whereby you set one or more fishing lines, baited with a variety of lures or bait fish behind the fishing boat which are then drawn through / dragged / tolled though the water. This causes the lure / baitfish to appear to be live and swimming which in turn attracts the attention of the predatory fish you are after and increase the chance of a strike.
There are various different trolling layouts which depend on the type of boat you have and most importantly the size (width) of the boat. The larger the boat, the more space you have to set up multiple rods & multiple lines.
The more lines you are able to set, the greater the variety of lures you can drag / troll behind the boat. If you set multiple lines with multiple different lures, this can be seen as a small school of fish and stand a better chance at attracting the attention of the fish you are after and better your chance at getting a strike.
In the attached diagram you can see a typical layout which varies from boat to boat. When setting lines they should be offset from one-another and not the same length as they can otherwise cross over and get tangled when the boat turns. Line layouts have certain names again depending on what your boat sizes is, rig layout and what you are fishing for.
Some boats come equipped with outriggers which allow for lines to be set wider apart and away from the boat to one or either side which can also be set longer.