Its BTs Voip phone software (a phone that uses the internet to make calls). Softphone is a virtual phone on your PC and the BT Broadband Talk that wants to configure your BT Voyager will let you connect a phone to it use.
It lets you make free calls between users and can work out cheaper to "normal" landlines. If you've heard of Skype its the same sort of thing.
I get free evening and week end calls through my BT broadband talk. I use the BT Home Hub but haven't set up a physical phone to it yet.
You don't have to use it, it comes as standard on most BT packages. You'll have a separate pone number for it, so depend on how keen you are on using Voip.
Some links below;
http://www.productsandservices.bt.com/consumer Products/displayCategory.do?categoryId=CON-INT ERNET-R1
http://en.wikipedia.org/wiki/Voice_over_IP