Notans are "value maps" of or for a painting. The idea behind them is to map the light and dark areas of an artwork. This is one of the most important parts of designing an artwork as it effects the impact of the artwork on the viewer.
Note in the example below, there is very little detail and the light and dark areas are shown as shapes. Usually notans are done in a few minutes, an artist might do several of the one painting. Colours can also be "mapped in this proccess, however, the real objective is to show the light and dark.