The public description is what other users will see as your description when they view your profile. Put things in here that you think other users may be interested in knowing about you. As with any online forum, putting personally identifiable information about yourself, such as your email address, phone number or home address is not advised. This will be public information. Some examples of things you may want to include:

I am a sports fan.
I am interested in politics
I am concerned about environmental issues.