On the home page, there is a "Your Weekly Mood" graph. Each day of the week shows a progress bar. There are 5 mood states ranging from "Stressed" through to "Happy". The progress bar will be complete when you record your mood as happy. The progress bar will show reducing levels of completeness depending on how close you are to a happy mood state.
This is just a quick way to log your current mood state without having to go to the Mood page where you can record your current mood and review your past mood tracking entries.