A Water Vapor image is an infrared satellite image that shows areas of moist and dry air. The moisture is indicated by the presence of water vapor (water in its gas state), which is invisible to the naked eye, but shows-up clearly on water vapor images produced by a satellite's infrared sensor and computers.
How does it work? The sensor focuses on radiation at specific wavelengths that is absorbed by the atmosphere. The absorbed energy held in the water vapor is detected and the computer goes to work painting a computerized image of the water vapor, which comes-out looking a lot like an image of cloud cover. By looking at a water vapor image, it's easy to tell where the atmosphere is moist or dry, and whether moist or dry air is feeding into a region. This can be a key in determining how much rain a front is likely to produce over a given region.