Spoonplugs are metal swimming lures that come in a series designed so that each size runs in a precise depth range. The smallest Spoonplug is 1.5" and runs 2-4' deep. The largest Spoonplug is 5" and runs 20-25' deep. In between are 5 sizes that run in increments of 4-6', 6-9', 9-12', 12-15' and 15-20'. Spoonplugs were created about 40 years ago by Buck Perry, who is considered to be the father of modern structure fishing.
As Buck Perry would say, there is no magic to Spoonplugs or any other lures; they are simply tools to help find and catch fish. When used in a systematic way that relies on trolling, sometimes quite fast trolling, Spoonplugs strain the water and are still the best tools I know for the "find" part of catching fish.
For more information go to buckperry.com. Spoonplugs may be purchased at that web site, but I recommend against trying the lures until you learn how to use them. Get Perry's book, "Spoonplugging: Your Guide To Lunker Catches". The book was first published 40 years ago, but it is still the best source I know of for freshwater fishing knowledge.
Spoonplugs and books are often offered on EBay as classics and collector items. Beware that EBay prices regularly are a lot higher than new items from the buckperry web site.