What is a zero-sum game?

A simple definition is 'winner takes all' i.e. a competition/conflict in which one side can only win at the expense of the other. In a cup semi-final, for example, only one side can progress to the final and the same principle can be applied in other fields. 

The 'zero-sum' is a mathematical reference to the fact that no additional value is added - one side must take from the other.

For a more detailed explanation of the concept, see this video: