Cold front refers to a weather condition occurring at the earth's surface level when the leading edge of a mass of cooler air replaces a mass of warmer air. The warm air is less dense with lower atmospheric pressure. As a result cooler air which is dense comes in to the areas of warmer air. However, less dense warm air is not able to mix with the denser cool air, and rises higher this phenomenon causes the the shape to cold front to become like a wedge.
While the weather condition before the cold front is characterized by higher temperature only, the rising warm air during cold front causes it to cool. This results in formation of clouds and showers, particularly when the warm air has high humidity.
Additional information on cold fronts and warm fronts are available at the website referred below.