The warm front refers to the region, where the cold air mass is readily being replaced by the warm air mass. This warm front is usually identified by the semi-circles in a weather map.
In the given image, a warm front is being shown. Here, the region comprising letter A is the warm zone and the region above the front comprising the letter B indicates a cold zone. The position A forms a low-pressure zone, so it is comparatively warmer than the position B.
