When someone links to something on twitter, either by embedding something or just pasting a URL, twitter will front it with its own t.co link. This means that you cannot verify what the URL is until you click it and your browser goes to the end result via t.co. I only really noticed this properly when my DNS sinkholing server (Adguard home) started blocking t.co links and I was getting an error when say, clicking a linked news article.
