A core concept in Meteor is reactivity. Basically, this means that Meteor can re-run computational functions automatically whenever their (reactive) data sources change. Typically, this happens without your having to think about it, since you’re using an in-built reactive data source, such as database collections. A typical example of a function needing to react to a data source (e.g. a collection) changing is a template helper, which needs to produce a new value to base the template rendering on.