What is what does a white check mark mean on messenger?

A white check mark on Facebook Messenger usually appears beside a sent message, indicating that it has been delivered to the recipient's device but not yet read. Once the recipient reads the message, the white check mark will be replaced with a blue check mark. If a message has a white circle with a blue check mark, it means that the message has been sent but the recipient has not yet enabled the "read receipts" feature. Additionally, if a message has a grey check mark, it means that the message has been sent but it has not yet been delivered to the recipient's device.