Read-out protection (RDP)
A lock in the microcontroller that stops firmware being dumped through the debug port.
Without it, anyone with a debug probe and five minutes can copy a product's firmware over SWD or JTAG. So nearly every microcontroller has one. On STM32 it is called RDP and comes in three levels: 0 is open, 1 blocks flash reads while a debugger is attached and mass-erases the chip if you try to go back, and 2 shuts the debug port for good. Nordic's equivalent is APPROTECT; AVR calls them lock bits.
The lock is only as strong as its silicon. Researchers have bypassed RDP on several STM32 families and APPROTECT on the nRF52 with voltage glitches during boot, a textbook fault injection attack. It raises the bar; it does not make firmware secret.