Value is how much something is worth. Often the best way to find the value of something is to use the price that it can be sold for. However Oscar Wilde wrote that 'people know the price of everything but the value of nothing'- in other words true value does not depend on money alone.
In math, a value is a number which is concrete, something everyone can agree upon. However people may disagree on the value of water, depending if you live in a desert or next to a river. Disagreements on the value of things can create fights between nations, political parties, religions, etc.