Decoction mashing is a traditional technique in central European brewing which produces the deliciously malty flavors for which German beers are best known. (It can also get lovely results in other malt-forward styles such as Scottish ales.) While many German breweries have been moving away from using it, especially for their palest beers, on a homebrew scale it is an easy way to impart this flavor into your beer. The essence of decoction mashing is to boil some of your mash, grains and all, and then combine it with the rest (which has not been boiled) to reach desired rest temperatures. It's common to repeat this process two or three times to reach several desired temperature ranges.
Unfortunately, most homebrewers are put off by decoction mashing, and for good reason - traditional decoction mash methods are challenging and very time-consuming, and abbreviated ones don't cook quite as much of that delicious melanoidin flavor into the beer, effectively compromising between a decoction mash and a more typical infusion mash.
The Schmitz process is a relatively modern variant on the decoction mash that develops a very full flavor while being faster and easier than a classic double or triple decoction mash. As long as you are willing to stand by the stove stirring for a while, this is your path to classic German styles.