how would you explain reshare on instagram stories to a client?

I'd keep it to this: a reshare is taking a feed post, or a story someone tagged you in, and putting it in your own story so your followers see it for 24 hours. For a 2,000 follower account it's worth it mostly for the second part. When a customer tags them, resharing it costs ten seconds and it's the closest thing to a testimonial they'll get on Instagram. Resharing their own feed posts to stories is fine too, just because a lot of followers only ever watch stories and never scroll the feed. One thing to tell them: you can only reshare someone else's story if they tagged you. Otherwise the option isn't there.