A registration key is a small file (usually named wincmd.key) that unlocks the full version of the software. While Total Commander allows you to use the program for free indefinitely for evaluation purposes, it is technically shareware. Without a key, you are greeted with a "nag screen" upon startup, requiring you to click one of three buttons (1, 2, or 3) to continue.
With a legitimate , you are entitled to free updates up to version 10.x (or later, depending on when you bought it—point updates are always free, and major version upgrades cost a small fee after ~5-6 years). With a stolen key, you cannot upgrade safely, leaving you on outdated, vulnerable versions. total commander registration key