One or two checks will appear in your sent message boxes within your conversation in a One-to-One chat or Group. A single check means that your message has been delivered. This also means that others in the chat have recieved a notification of your new message, if they have notifications turned on. Two checks (in the One-to-One chat only) means it has been read by the other member in your chat. An exceptionally long delay in the appearance of the second check could mean that the person has not yet clicked into the app and read your message.