if i play two spells in reaction to themselves, so two on chain. the 2nd spell counts a played first since its resolved first for heron. but then the discount says my next card is discounted. does that mean the first card on the chain which has not resolved yet will be my 2nd card played? or the next play from hand? how does this discount work here?

No — the card still sitting on the chain doesn't get the discount. The discount only applies to the next card you newly play after the trigger resolves.

You're right that the reaction spell resolving first counts as your "first card played" (FAQ #11688), so Heron triggers when that one finishes resolving. But Heron discounts "the next card played (added to the chain and queued for finalization) this turn after the ability resolves" (FAQ #11762) — your first spell was added to the chain long before the trigger resolved, so it's exempt. It resolves at full price, no matter that it's still on the chain when the discount appears.

So the sequence is: spell B resolves → Heron triggers → trigger resolves, creating the discount → spell A resolves undiscounted → the very next card you play from hand this turn (or any Reaction you squeeze in right there, before A resolves) gets the [2][A][A] reduction. Note it's "this turn" only — if you don't play anything else before the turn ends, the discount expires.