This means that when a user writes out their handle, hashtags, URLs or stock tickers, Twitter will instantly highlight the text to indicate that they are linked and clickable.

Users can also right click on highlighted words as soon as they're done typing and hit "Open in new tab" to use them as hyperlinks before the tweet is sent out -- this doesn't appear to work for URLs, though.

Twitter Starts Highlighting Handles and Hashtags as They Are Typed [latimes.com]