Decoction mashing is a traditional German brewing technique: pull a portion of the mash (grain and liquid together), boil that portion separately in a second vessel, then return it to the main mash — the returned boiling liquid raises the whole mash's temperature by a controlled amount, letting a brewer step through multiple rest temperatures without any external heat source touching the main mash tun directly.
Why brewers still bother with something this labour-intensive
Boiling a portion of the grain itself, not just the liquid, develops melanoidins — deep, malty, lightly caramelized flavor compounds — more intensely than a standard infusion mash produces, which is a real, tasteable reason some traditional German lager styles (particularly Oktoberfest/Märzen and Munich Helles in their most historically faithful forms) still call for it rather than treating it as an obsolete curiosity. It developed historically as a way to hit multiple mash temperatures using only kettle heat, before precise external temperature control of a whole mash tun was practical.
A modern step-mash gets close without the extra vessel
Most homebrewers today reach for a step mash (heating the whole mash directly, in stages) rather than true decoction, since it reaches similar target temperatures with considerably less equipment and time — full decoction remains a deliberate choice for brewers specifically chasing the extra melanoidin depth it develops, not a step anyone stumbles into by accident.