The Earth is round and completes a full rotation within one day, that is to say in 24 hours. The Sun therefore only shines on one part of the Earth for a certain length of time, our daytime. It is then dark on the opposite side of the world, their nighttime.
Because everybody wants to see the Sun during the same daytime hours the world was divided into separate time zones. There is a one hour’s difference between the various zones. If you fly to China for example, you have to travel through several time zones and therefore have to put your watch forward six hours. China is actually so far away from Germany that it is already the evening there while we are having lunch!