What does air humidity 100% mean?

100% humidity is also called "dew point". The water content in the air is different at different temperatures. For example, someone asks you, is the air in Antarctica dry or the air in the Sahara desert dry? Most people will choose the Sahara desert, but the correct answer is Antarctica! Because of the high temperature in the Sahara desert, the air can contain a lot of water and can't reach the "dew point", so it doesn't rain.