As we already saw throughout the book, Meteor uses something called reactivity.
One problem that a developer has to solve when building a software application is the consistency of the data represented in the interface. Most modern applications use something called Model-View-Controller (MVC), where the controller of a view makes sure that it always represents the current state of the model. The model is mostly a server API or a JSON object in the browser memory.
The most common ways of keeping consistent interfaces are as follows (courtesy: http://manual.meteor.com):
Poll and diff: Periodically (for example, every second), fetch the current value of the thing, see whether it's changed, and if so, perform the update.
Events: The thing that can change emits an event when it changes. Another part of the program (often called a controller) arranges to listen for this event, gets the current value, and performs the update when the event fires.