How is weather related to the jet stream?

The Jet Stream is the current of circulating area that surrounds the entire planet. Through convection, the jet stream is able to disperse heat energy and help to keep temperatures moderate. Without it, parts of the planet would be significantly hotter and others significantly colder. There would also be a much more dramatic difference in temperature between the side of the planet experiencing day and the side experiencing night. Places like mars, which lack both atmosphere to insulate and a jet stream to help disperse can vary by hundreds of degrees between night and day.
The jet stream travels from west to east. As a result, Maritime, tropical, and polar air masses are all pulled the way it is moving. This influences the weather patterns because the paths air masses follow are forged by the jet stream's movement.