On each card there is a "play" button, & a "delete" button, but what does the green check mark mean?

thanx

The green check mark enables or disables the card to be available to the GUI to play.

In other words, if you check the mark to red and then turn the GUI "on" it will not play that card but will play any of your others that are check marked green.

You will know it is red because the card will "grey out" slightly from the bright color that the rest of the cards are.