Co-evolution means reciprocal evolutionary change in interacting species. It is a form of reciprocal evolutionary change in interacting species where the mutual evolutionary influence between two species increases the dependencies between both until they become more or less dependent on each other. An example are insects and the flowers they pollinate: both dependent on each other in order to survive.
In other words co-evolution describes the interdependent evolution of two or more interacting species, each in response to the other, in which the evolutionary changes of each species influence the evolution of the other species. A typical example for co-evolution is the Red Queen Effect and evolutionary arms races.