Ranking Terms is measured as the quantity of unique search terms that you website ranks for.
SEO Tools like Ahrefs use this metric and its useful to understand how many unique search terms a website ranks for.
However, its important to note that ranking terms doesn’t account for the position. If you have 1000 ranking terms, but the average position of those ranking terms is on Page 10 of Google, then that isn’t going to drive much traffic vs 10 ranking terms on Page 1 of Google.
Ranking Terms is useful to measure alongside a content strategy as naturally as you create more unique content, your website will increase its  visibility on ranking terms.