The ITCZ is a zone of convergence at the thermal equator where the trade winds meet. It is a low pressure belt and migrates with the changing position of the thermal equator.
The thermal equator receives the most intense heat from the Sun. Around 20th June each year the Sun is overhead at 23½º North, the Tropic of Cancer. Around 20th December the Sun is overhead at 23½º South, the Tropic of Capricorn.
The movement of the thermal equator shifts the belts of planetary winds and pressure systems to the north and to the south annually, as the diagram below shows.
To understand the cause and impact of the ITCZ we must first look at the trade winds and air masses over Africa.
The north-east trade winds come from the Tropical Continental (cT) air mass.
The south-west trade winds come from the Tropical Maritime (mT) air mass.