TL;DR means “too long; didn’t read” in texting, where it can be a response to a text message or a summary of a long text message that you wrote.
When people respond to a long text message with TL;DR, it’s usually a form of criticism. A more polite response is, “I’m sorry. I don’t have time to read this. Could you tell me the short version?”
When you write a very long text message, you can end it with a TL;DR to acknowledge your own verbosity and/or show respect for your reader’s time (e.g., TL;DR I have tried that new ramen place, and it’s amazing but pricey).
In text messages, TL;DR has capital letters. The QuillBot app will automatically capitalize initialisms like TL;DR when you type them in lowercase.