ITCZ stands for Intertropical Convergence Zone. It is a region of clouds, rain, low level convergence and rising air. Portions of the tropics that have a wet season have the wet season when the ITCZ is overhead or nearby.
The ITCZ is a global belt of low pressure produced by the convergence of air coming out of the high pressure belt to the north and the high pressure belt to the south. The portions of the tropics dominated by high pressure have very dry weather and many of the world's deserts are located under this region.
The ITCZ shifts position over the course of the year since the sun's direct rays on the earth vary over the course of a year. The ITCZ tends to be located under and near where the sun's rays are most direct. Thus, the ITCZ will be located north of the equator in the Northern Hemisphere summer and south of the equator in the Northern Hemisphere winter. However, the mean or average position of the ITCZ is located north of the equator.
The ITCZ has a mean position north of the equator because there is much more landmass in the Northern Hemisphere as compared to the Southern Hemisphere. Look at a globe and notice how much more land there is in the tropics in the Northern Hemisphere. Land heats more than water. When the land surface is warmer than the water it promotes a large scale sea breeze (flow from ocean toward the land). This produces a convergence of air over the land. Areas such as S.E. Asia have heavy rain in the summer due to this circulation. The ITCZ has its most northerly displacement in the summer over Asia due to the large landmass of Asia to the north of the equator.