Query volume
Also: search volume
How many times a month people type a query into search.
Volume shows demand: 50,000 impressions a month versus 200. You check it in Yandex Wordstat and other tools.
High volume does not equal traffic: out of 50,000 impressions, a couple of percent at most will reach your site for that phrase. It is often smarter to aim at mid- and low-volume queries, where competition is lower.
Full explainer
Query volume: what "50,000 impressions" means →