What is the meaning of "21 speed" gear bicycle or Mountain Bike?

A 21 speed bike has 21 gears but effectively only 9 speeds. You can cross chain the bike to some extent to get more variations, but if you do that it wears the chain out quickly which will cause shifting problems. Also, the wider your rear cluster, the more you need to favour the middle gears in order to prevent chain wear (this becomes a big issue when you get to ridiculous 27 or 30 speed bikes). The upper and lower limits are determined by the gear ratio at your highest gear (biggest gear on your crankset, smallest sprocket) and that of your lowest gear (smallest crankset gear and largest sprocket).