Your research does not have to win a Nobel prize to be a PhD. Despite this, students do worry about making a significant and original contribution, being good enough, and being able to judge if their work is worthwhile.

Indeed, 'an original contribution' is a difficult concept to define and, if you look in your university's publications, doubtless you will find it alluded to or mentioned in different contexts. But, when we think about it, it is difficult to capture and express succinctly. For example, the Griffith University Handbook (p.41) says that the work must:

make an original and significant contribution to knowledge and understanding in the relevant field of study as judged by independent experts applying accepted contemporary international standards.

And one of the statements made about it in the University of Queensland Doctor of Philosophy Handbook (page 1) reads:

The doctoral thesis provides evidence of a contribution to knowledge with a level of originality consistent with 3-4 years of full-time study and supervised research training. It also demonstrates a candidate's capacity for critical analysis and that he/she is capable of pursuing scholarly and programmatic research that answers significant questions within a 3-4 year time frame.

Careful selection of your topic, the design of your study, close discussions with your supervisor and other reputable researchers in the discipline, and wide knowledge of your field should ensure that your work meets the set requirements.

So, the important thing is that, in your writing, you explicitly indicate the significance of your work and therefore your own contribution to the discipline.