Answer
It means that, together, they have all the outcomes of a sample space, but they have no outcomes in common (disjoint events).
Work Step by Step
Suppose that a sample space, denoted S, has 100 outcomes. Let E denote an event that has 20 outcomes. The complement of E, denoted $E^{c}$, has the other 80 outcomes of the sample space S. So:
$P(E)+P(E^{c})=1$
