This air mass originates in the Atlantic Ocean/Gulf of Guinea. In tropical latitudes this air mass is hot to very hot, with high relative humidity, bringing unstable weather.
This air mass originates in the Sahara Desert. In tropical latitudes this air mass is hot to very hot, with low relative humidity, bringing stable weather.
When these two air masses meet, moist air is forced upward. This causes water vapour to condense as the air cools and rises, resulting in a band of heavy precipitation around the globe.
As the ITCZ moves north it carries the mT winds over the land. This will bring wet weather. At the same time places to the north of the ITCZ will be experiencing hot dry weather, under the influence of the cT winds. Thunderstorms regularly occur directly beneath the ITCZ.
Variation in the location of the ITCZ dramatically affects rainfall in many equatorial nations, resulting in the wet and dry seasons of the tropics.