means that your message has been sent from your device

but it has not been received

by the person you are messaging yet.

A second gray check mark

means that your message has been delivered

and received on the recipient's device.

When the two check marks turn blue,

that means your message has been read

by a recipient with read receipts turned on.

Some people have read receipts turned off.

If that's the case, they could have read your message,

even if the check marks remain gray.