TSB (Training Stress Balance) is just CTL - ATL and it's intended as a measure of form or readiness to put out best efforts. During training it's normally negative since you should be ramping up CTL and ATL but ATL climbs faster. When you rest or taper for an event CTL drops more slowly than ATL and your TSB rebounds towards or into the positive numbers. You should expect to see best performances during these rebounds. TSB is a good way of visualizing how deep a "hole" you're digging during hard training periods and how much you rebound during rest or tapers. It can be a useful metric for planning training, rest and for peaking for key events.
Here's an intro by Hunter Allen:
http://www.swimsmooth.com/Documents/Performance_Manager.pdf
-Dave