What's a Lookup? It is a general registry permitting clients to find instances of services (implementation of a given interface). To put it another way, it is a mechanism by which we can publish various artifacts, and other parts of the system can look up these artifacts by a key (either a Class or a Lookup.Template, which we'll not discuss here), with no coupling between the modules.
This is often used, as we'll see, to look up the implementations of a service interface. Do you recall earlier when I mentioned that often we see APIs defined in one module and implementations in another? This is where that comes in especially handy. Suppose you're developing an API to retrieve photos from an online service (which would be a great feature for this application!). You plan to deliver an implementation for one service, say Google Photos, but want to enable a third-party developer to provide an implementation for, say, Flickr. If you put the required API interfaces...