Today I want to talk a bit about Lightning Component Facets. What is a Facet? A facet is an attribute that we can define and place in a subcomponent, in order that another container component, which references the subcomponent, can fill up the facet space with some html code. Note that the subcomponent and container component are simply components, but I will call them in this way in order their relationship is easier to understand .
A facet attribute is similar to any other component attributes, but instead of having a primitive, collection, sObject or custom class type, it has an “Aura.Component” type. And instead of holding those kind of data, it will hold HTML markups or even another component.