Flash offers are offers that appear on the Checkout 51 offer list for a limited amount of time instead of the usual Thursday-Wednesday Checkout 51 cycle.
If a flash offer is added, members will receive a notification letting them know. (So make sure your notifications are turned on!)
Besides the time frame, flash offers are just like regular offers and should be claimed in the same way. As always, be sure to check the details for product specifications and expiry.