Fasting is a word which represent the capacity of the human body to run without any external food using its own natural energy reserves.
It can be a very useful skill and asset for nomads especially in certain conditions (wilderness, expensive places, unsafe food, etc) where is hard to get good food in an easy way, whether it may be asking, dumpster diving, buying or shoplifting.
It is as simply as it sounds! JUST DON'T EAT! :) Human body can run up to 30 days without introducing any source of ordinary food and energy. It's healthy to fast one day a week, if you want to fast longer period the best is always to increase little by little. You need also some reserve (fat) in order to fast for more that few days, otherwise nutrition could be taken from muscles which may weaken.
Skip a meal
It is the simplest and easiest way to perform fasting. We are used to think that we need to eat 3 times a day which is totally not true. Between every meal it changes just the lenght of the period you don't it in between (breakfast, means exactly breaking the night fasting..), getting them longer won't be as hard as you imagine. When the hunger come, just ignore it, don't think to it and in less than 20-30 minutes it should have been disappeared (unless you keep thinking stuff like: "gosh, i'm really hungry, how i'd love a gorgeous vegan healthy free food right now..")
Fast as medicine and health improvement
In several kinds of diet fasting seriously (from 1 to 3 days to 1 week or longer period) is more than recommended to cure intestine and stomach diseases as to keep them healthy and strong. While fasting your body get disintoxicated from all toxines it can't get rid of while you keep eating. It is often recommended to drink a lot while fasting intentionally as it helps the body get out toxines, though there is also a 'dry' kind of fasting which consist in not drinking either (as muslims do in daytime during ramadan) but which should never last more thna 24 or exceptionally 48hrs.. Consult a real and serious doctor (not normal big pharma cheaters..) about it or ask people with experience on it can be a help to the main info you find on the internet.