For me, basic prepping and survival comes down to 5 things:

1)  Water

2)  Food

3)  Shelter

4)  Fire

5)  Light

Sure, there are other things you'll want to have when preparing for a survival situation...first aid, medications, guns and ammo among other things.  But at the end of the day, prepping water, food,shelter, fire and light will serve your core needs, equating essentially to the bottom tiers of Maslow's Hierarchy. 

The first two, water and food, you will die without very quickly.  Dehydration can kill in as little as 3 days.  Starvation takes a bit longer depending on metabolism and activity levels, and many folks could likely go 3-4 weeks without eating, although you will continually weaken without nourishment.

Shelters' necessity level depends a good deal on the climate you live in.  While it may not be necessary for a while in mild climates, hypothermia can kill faster than even lack of water.

Fire is the basic and most important tool of mankind, offering warmth (shelter), light, the ability to cook out many of the impurities in food, pasteurization of water, and a hundred other uses.  While fire may not sustain life, it is invaluable in preserving it.

I often debate with including 'light' on my list, and I can see reasons for not including it.  However, I usually do because light is an intangible benefit for humans.  Without it, we are helpless for much of the day (night).  Our ability to create light is one of the differentiators between man and animal, as we are not required to live by the cycles dictated by the sun to many other creatures.

So whether you are just starting your prepping and survival plan, make sure you have the 5 bases covered, starting with water.  I know many individuals and groups who skip around and prep haphazardly (At one point I was one of them); however, working without a plan, and missing any of the above could cost you dear.