Optics in a Digital SLR

Ok so I'm thinking about getting a Digital SLR camera. I don't understand. I definitely want a Nikon. My dad keeps telling me that it has to do with the optics, but how do I know that it has good optics?

Added (1). What is a good lens? I'm 16 I don't understand that's what I need your help for

The optics on a DSLR is the lens you have attached. If you want really good photos, you will need to use a good lens.

Read reviews online