A feature of StackExchange chat, "oneboxing", automatically inlines some links that are posted on a single line by themselves.
Some links will be automatically inlined if posted on a single line by themselves, such as:
- Stack Exchange questions, answers, and users
- Chat messages and rooms
- Wikipedia pages
- Youtube videos
We call this "oneboxing". For example, entering the following in chat on a line by itself …
… will produce a onebox for that link, like so:
For more information on what links produce oneboxes, see Which links and sites are handled specially in chat?