what exactly is local seo citation?

Any mention of your business name, address and phone number on another site. Yelp, Bing Places, Apple Business Connect, industry directories, the local chamber of commerce site, that sort of thing. The point is consistency. If your old address is still on half of them or the phone number is written three different ways, it muddies things. Get the main ones right before chasing hundreds of tiny directories, those add very little now.