A Mega Backdoor Roth lets you put after-tax dollars into your 401(k) above the normal employee deferral limit of $24,500, up to the much higher total annual limit of $72,000 that covers employee plus employer contributions. Those dollars go into your Roth 401(k) account, so all future growth comes out tax free.
Mega Backdoor Roth is a plan-level feature your plan admin enables after a compliance review. It is not something an individual toggles on. When enabled, it appears as its own contribution bucket.