If something is necessary, it is very important, otherwise something will be impossible, or it will not work as it should do. The opposite is unnecessary.
- To travel from Europe to America it is necessary to cross the ocean.
- A car is necessary for my work. I can not do my work without a car.
- There is no need to do that. It is quite unnecessary.
Something that is necessary is a necessity. The "bare necessities of life" are things like food, clothes and shelter (things you need to keep alive).