This command exists in Smoothieware purely for compatibility with other firmware (like Marlin/RepRap firmware).
When received, Smoothieware acknowledges the command but does nothing.
Smoothieware automatically initializes the SD card on boot, so manual initialization is not required.
The command is used as such:
M21
Which means: the command will be received but Smoothieware will perform no operation.
The M21 command requires no parameters.
If you command:
M21
Smoothieware will acknowledge the command but perform no operation.
The SD card will remain in its current state.
The M21 command requires no special configuration settings.
Since Smoothieware handles SD card initialization automatically, this command is not necessary for normal operation.
These resources are used as references: