The tweak means you no longer have to get out your phone and retype a number when someone sends you one in an e-mail or chat message. All you have to do is click the number, and Gmail's dialpad will pop up with the number you're trying to call. Click Call and you're on your way. A little green phone icon next to numbers in your contacts will allow you to do the same thing.
If you don't already have Google's voice and video plug-in installed, you'll be prompted to do so first. As a result, the change works as a small advertisement for Google since many users will likely click the number out of curiosity to see what happens.
Late last year, Google announced that it was extending free calling to all of the US and Canada until December 31, 11:59PM 2011. Calling in Gmail is currently only available to US based Gmail users. You can start using it on Windows, Mac, or Linux by navigating to Google.com/Chat/Voice.