Gif: Global Total Precipitable Water and Anomaly, January 5-6, 2017
The atmosphere has an enormous amount of moisture that circulates around the globe. Not all of it however actually condenses into rain, sleet, or snow since the right balance of pressure and temperature are needed to create precipitation. The dataset shows the total precipitable water (TPW) content in the atmosphere. In other words how much moisture is available to theoretically precipitate into rain, sleet, or snow.
TPW is measured by a variety of satellites. In this case the data from the AMSU and SSM/I microwave sensors are merged together to create a daily composite of global TPW. The values are expressed as "precipitable millimeters" or "kilograms per square meter." Both refer to an amount of water over a defined area.
Significant features that can be identified in TPW data are the atmospheric rivers that flow off the oceans and onto coastal areas. A famous example is the Pineapple Express that forms in the Northeast Pacific and impacts the Northwest US. Notice how TPW values are much greater over the equator and ocean. High levels of evaporation in these areas are one of the primary drivers of atmospheric circulation.