It depends on your role and what you've worked on. For my former role, it was really easy to quantify what the projects I worked on did, because they were purely internal, I was a lead on them, and they were almost entirely geared toward saving labor hours. It was easy for me to say "Minimum, this saved the company $60k".
One of the things I've never seen many technical people do is track cost/benefit metrics. Learn to spot how you're impacting the company financially, and, even better, if you want to start pitching projects and ideas to the people above you, learn how to do RoI and IRR calculations (they're honestly really simple math). That's neither here nor there, though.
Keep a record (written in a notebook, on Google Keep, whatever works best for you) of what you've accomplished since your last review. If you don't have anything groundbreaking, that's ok - show that you contributed to a solid product or stable business.
