The Napkin Math Tool is a simple estimation method that allows users to perform quick calculations to gauge time or resource requirements related to everyday scenarios, such as estimating the time it takes for a group of people to complete an action in sequence. For instance, in the example given about a high school group microwaving meals sequentially, the napkin math method uses a basic addition approach to provide rough estimates, which can lead to surprisingly accurate results when cross-verified with conventional calculations. Users should remain mindful of the units involved in their calculations to ensure accuracy in the estimations.