Refer to figures.
A "warm sector" is the air mass that exists after a warm front, but before any pursuing cold front. This is a part of every polar front depression (travelling low pressure zone) that forms in the mid latitudes. As polar front depressions are made up of different temperature air masses, one of them (which comes from South of the Polar front), is the warmest, known as the "warm sector".

This warm sector is encapsulated at the front by the warm front, which is where the warm front pushes the cold air along, and from behind by the cold front, which subsequently pushes the warm air along (at a higher speed). The cold front moves approximately twice as fast as the warm front, so the warm sector is slowly eaten up until it disappears completely, an occluded front forms, and the warm sector air is forced off the ground and into the upper atmosphere.

Warm fronts are denoted by red lines with semicircles indicating the direction of travel, and cold fronts are indicated by blue lines with spikes indicating their direction of travel. Occlusions are usually purple with both semicircles and spikes.

We can see from the figure that the location of the blue dot exists after the passage of a warm front, but ahead of a cold front which has yet to pass, and is therefore within the warm sector. The warm sector tends to have good weather in summer, with some scattered Cu, but in winter it often has lower stratus clouds and drizzle.