Horizontal discovery helps you scan the infrastructure and store in cmdb it is not concern about how two CI may be related.
Top Down discovery done after horizontal discovery . It relates two CI. For example a web service individually can hosting server, load balancer, data base and so on. All of them are discovered with the Horizontal discovery. Now you want to know for a particular service what all CI's has been used and how they are related, we use top down mapping.
If you want to start then a now learning course for discovery you can do. It will give you basic idea. Also I would recommend to check for now learning CMDB course. Once both are done you can go with service mapping. Best of luck.
