Opportunity Products are used to calculate ARR, MRR, and TCV on your Opportunities. They appear in the 'Products' section in the top left hand corner of your Opportunity page.

Because Products are used to calculate these metrics, it's important to add them to Opportunities to get an accurate view of your pipeline. This workflow lets you require that at least 1 Product is added to an Opportunity before it advances to a certain stage of the sales process.