Turbocharger (VGT)
A turbocharger uses exhaust gas to spin a turbine, which drives a compressor that forces extra air into the engine. More air means more fuel can burn, which means more power.
Most modern diesel pickups use a VGT, a variable geometry turbo. Movable vanes change the exhaust flow to give quick response at low RPM and big airflow at high RPM. Soot can gum up those vanes, causing underboost codes and power loss.
Listen to your turbo. New whistles, loss of power, or excessive smoke are early warnings worth investigating.