Change in weather occurs because air in the atmosphere is constantly circulating and moving. Air masses have similar temperatures and humidity levels which are determined by where they were formed.
Frontal boundaries form when two air masses meet. Cold fronts on a weather map are blue lines with triangles. Warm fronts on a weather map are red lines with semi-circles.
Cold Front occurs when the cold air moves under warm air and pushes it up where it cools and condenses, forming clouds. Big temperature differences between the warm and cold front could result in severe weather.