The red exclamation mark symbol in telegram is used to show you the error white you are sending a message to someone on telegram. It denotes that the message you are sending has failed even after waiting for 5 minutes.
You can just click on the symbol and resend the message to the person or you can try after some time. There are many such symbols that might be helpful for you, read our blog to know all about them in detail
Read more: https://ityug247.com/icon-and-symbols-on-telegram