Hello Simer,

PhD (Doctor of Philosophy) is a postgraduate degree, and it culminates in the writing and defense of a thesis. A doctorate thesis should contain original work, which means that it should contain some novelty to your field, that is, the thesis makes a completely new contribution to the field. I wouldn't use the word invent, because not all fields permit the invention of something new.

The PhD usually consists of some coursework on the field, for deepening knowledge, but the most substantial part of it is research. Different fields have different types of research; a pure Mathematician might read a variety of papers and work on novel derivations/proofs, whereas a Biologist will spend most time in a laboratory, conducting actual experiments.

Hope this will help you.