Fasting is the act of abstaining from food and/or drink for a period of time.
Fasting is entirely optional. In fact, the American Heart Association recommends a non-fasting lipid test as a more convenient option for patients.
Non-fasting glucose provides an initial screening value that can then be followed up with by a doctor, who may recommend another fasting glucose measurement, three-month A1C, etc.
If you do choose to fast, please make sure to avoid eating or drinking anything besides water for 8 hours before your screening.