Official Aviation Definition
A Flight Level (FL) is a surface of constant atmospheric pressure related to a standard pressure datum of 1013.25 hPa (29.92 inHg). Flight levels are stated in three digits representing hundreds of feet; for example, FL350 corresponds to a pressure altitude of 35,000 feet.
In Simple Words
Standard cruising altitudes used above the transition altitude where all airplanes set their altimeters to the exact same setting so vertical separation is guaranteed.
Real-World Flight Example
Transcontinental flights across the United States typically cruise between FL310 and FL410.