Topics are the logical “buckets” your events are split into. They serve to indicate the different types of events you’re sending, and act as the signpost for sending events to downstream subscriptions.

​
Summary

A topic, identified by it’s slug, is the destination you send events to. Subscriptions are then attached to a topic, and consume the events that their given topic has recieved. When sending an event, you send it to a single specific topic. 
client.publish("topic-slug", event);
The topic then distributes the event to all the child subscriptions for consumption in your application.