A downrigger (electric or manual) is a device to get your lure and line down deeper without adding weight to your fishing line.
It consists of a spool with a brake lever mounted on or near the gunnel that has approx 250 to 400 ft of wire or braid on it, connected to a cannonball which is the weight (12,15,18,or 20 lbs ).
You attatch your fishing line to it with a release clip and lower the fishing line and the downrigger line to the desired depth.
When a fish hits it will pull the fishing line out of the release clip and you are free to fight/catch the fish .