This is a debug tool with its own RAM in the cartridge.
As huey put it: "You can call it from basic with CALL MSXTRA or during boot time by holding CTRL + STOP pressed.
MSX-TRA can prove to be very usefull when your program contains a bug which makes the system hang completely. Just hit the reset button and hold CTRL+STOP pressed until you are in MSXTRA. Then you can start disassembling the program and check the memory contents. Since MSXTRA contains its own RAM, the debugger will not overwrite any byte of your program's memory."