Issuing an M24 causes the file selected with the M23 command to be printed. If the print was previously paused with M25, printing is resumed from that point. To restart a file from the beginning, use M23 to reset it, then M24.
When this command is used to resume a print that was paused, Smoothieware runs macro file resume.g prior to resuming the print.

The command is used as such : 

M24 

Which means: Print the file that was previously selected by the M23 command.

The M24 command requires no parameters.

If your SDcards contents was:

• config.txt
• firmware.cur
• Readme.txt
• myGgcode.gco

And you previously executed M23 myGcode.gco, then issuing the command:

M24

Will result in the gcode commands in the file myGcode.gco being executed.

The M24 command requires no special configuration settings

These resources are used as references:

• http://reprap.org/wiki/G-code#M20:_List_SD_card
• http://reprap.org/wiki/G-code#M-commands
• Go to other M-codes