Your position in Google's organic search results for a given keyword.
You either rank or you don't. Ranking position determines click-through: #1 gets ~30% of clicks, #2 gets ~15%, #3 gets ~10%, and it falls off fast. Page 2 basically doesn't exist; fewer than 1% of searchers go there. Your ranking for a keyword is what determines whether SEO delivers traffic.
Ranking is the goal. Track weekly. Know your top-ranking keywords and their trends. A drop from position 3 to 8 overnight is an emergency, not a 'we'll look into it' ticket.