What is what does it mean on snapchat when the delivered is flashing?

When the "delivered" message is flashing on Snapchat, it means that the recipient has received the message and it is still unopened. This flashing indicator is designed to draw the recipient's attention to the fact that they have a new message waiting for them. Once the recipient opens the message, the flashing will stop and the message will be marked as "read" on both the sender and recipient's devices.