The jet stream can be thought of as a river of fast moving wind high up in the atmosphere. In general, all wind is caused by a difference in pressure between two places. Think about when you open a can of soda, the air inside has high pressure so when you open the tab, air blows out really fast until the pressure is equal. The same thing happens in the atmosphere around Earth but on a much larger scale. The pressure difference of the air comes from uneven heating by the sun. Hotter air becomes higher pressure and colder air has a lower pressure. You can read more about it on a NOVA web page:
The Jet Stream