One gray check mark
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.