I agree with Phil. A lot of messaging apps have “read receipts”. On WhatsApp, you get one check mark when your message is sent, two check marks when your message is delivered to the other person (if the other person’s phone is turned off or not connected, the message stays in the server and you only get one check mark till their phone is reconnected), and blue check marks when the person reads your message. To be “left on read” means that you can tell the other person has read your message but hasn’t responded. A lot of young people nowadays interpret that as being ignored.
Some apps allow you to turn off this feature so that you don’t see when people have read your messages and they don’t see when you’ve read theirs.