How to enable EMERGENCY_PARSER in Marlin firmware?
The constant EMERGENCY_PARSER is located in the advanced printer configuration file Marlin/Configuration_adv.h:


// Enable an emergency-command parser to intercept certain commands as they
// enter the serial receive buffer, so they cannot be blocked.
// Currently handles M108, M112, M410
// Does not work on boards using AT90USB (USBCON) processors!
//#define EMERGENCY_PARSER


To enable the EMERGENCY_PARSER, you need to remove the // before #define EMERGENCY_PARSER and recompile the sources.

Normally your printer will execute a command until it is ready to accept a next instruction. Without the EMERGENCY_PARSER set, the printer finishes the instruction that it is executing at the moment, if set, the execution is interrupted and immediately sent and thus not waiting for a clear space in the buffer.