You could just Google it...but:
"A finsta is a secondary, usually private, Instagram account where users are more lax in deciding what they post. It’s a private space that might be more personally authentic and is shared with close friends. This is a contrast to someone’s main account, which might have more heavily curated content"
Basically an IG, sometimes private but usually just not overtly connected to someone's main account, where they post more candidly. Sometimes you get absolute gems of content from finstas because the celeb/influencer doesn't have to be as 'brand friendly'