"About me" is a feature displayed publicly on your profile. It is a powerful tool to advertise yourself to prospective clients. Giving clients some background on who they will be meeting with creates a nice personal touch and allows for clients to pick their best match.
Here are some topics/ideas to help you get started:
Work-Related:
Experience
Years practicing
Areas of interest
Niche areas
Personal:
Your own pets
Hobbies/interest
Your "About me" section lives in your account under "Personal Information".