I'm trying to understand how CQ form components works. I see that they use a variable called 'resource' a lot. For example, in the beginning of every componenet it always goes:
final String name = FormsHelper.getParameterName(resource);
final String id = FormsHelper.getFieldId(slingRequest, resource);
final boolean required = FormsHelper.isRequired(resource);
I know that Sling treats everything as a resource. But what exactly is this specific piece of 'resource'? Where is it defined? Where does it come from? And what does it contain?
The resource variable, which is an implementation of org.apache.sling.api.resource.Resource, is an object that represents a node entity in the jcr repository, but comes with some additional convenience methods compared to e.g. lower level javax.jcr.Node object.
In this case the mentioned resource likely represents the component's resource.
To explain why sling is using the terminology Resource:
A resource is a fundamental concept in restful APIs. Resources are typed objects with associated data, relationships to other resources and methods that operate on it.
Sling is actually a restful layer on top of a Java Content Repository. For the sling layer the repository is a virtual tree of resources. I highly recommend you to read the official documentation for more details on this topic https://sling.apache.org/documentation/the-sling-engine/resources.html