Here are all the different icons that may be visible in the icon tray (next to the clock in the lower right corner of your screen):
The logging is paused (but not saved).
BWTracker is not logging anything. This may happen when the evaluation period expires and you have not purchased a copy of BWTracker.
BWTracker is logging, but there is currently no traffic.
You are receiving, but not sending, data.
You are sending, but not receiving, data.
You are both receiving and sending data.
Note: In the Settings dialog you may specify the icon sensitivity. If you have set the sensitivity to a very high level, there may be data transferred without the icon blinking in the tray. BWTracker is still registering all the data transferred regardless of the sensitivity.