What does “Out of Play” or “In Transit” mean?

From the rulebook:

“To pay energy during your turn, move dice showing that much energy from your Reserve Pool to Out of Play. Those dice will go to your Used Pile during Cleanup. You can also pay energy during your opponent’s turn, but that energy goes directly into your Used Pile. You can’t choose to spend energy for no reason.”

When played on your turn Action Dice also go to “out of Play” when used.

This means that, on your turn, once you have spent an energy or used an action die it cannot be accessed for game purposes (filling your bag, for example) until the end of the turn.

Note: there is not an “Out of Play” or “In Transit” zone on any official playmat.