What does 18650, 21700 etc mean?

18650 etc refers to the size or measurements of a battery or more accurately a lithium-ion battery cell ie 18mm by 65.0mm or 21mm x 70.0mm

The 18650 is the most popular rechargeable battery in vaping. There are other sizes available like the 21700, 26650 and the 18350 to name a few. The battery life is determined by the mAh or milliampere hour