Conditions are the first half of a Rule execution. Every time an Event is received, Rules configured for that incoming Event's specified Source are each evaluated based on their configured Condition. If the Condition evaluates as true, the Rule's Actions are executed. If they evaluate as false, the Actions are ignored.
Conditions make it easy to only perform certain Actions when certain things are true. Conditions can be as simple as specifying the Event be of a certain type, or as complex as evaluating a Javascript function.