There are several blood tests that require you to fast beforehand. Fasting means going without any food or drink except water for a period of 10 - 12 hours. The minimum acceptable fasting time is 8 hours and the maximum time is 16 hours. Fasting for less than 8 hours or more than 16 hours can cause test results to be incorrect.
If you take medications on a regular basis you should ask your doctor whether you should take them during the fasting period or wait until after the blood sample is collected.