The guaranteed prize attached to the final pack in a run.
Some packs reserve a specific prize for whoever opens the last pack of that pack's run.
Last Card
A specific named card, reserved and held out of the normal pool. It cannot be pulled by ordinary means — it's guaranteed to the final opener.
Where a pack has one, you'll see exactly which card it is on the pack's page. It's not a mystery prize.
Last Reward
The same idea, but the prize may be a sealed product — a booster box or similar — rather than a single card.
How it's awarded
When the final pack of the run is opened, the reserved prize goes to whoever opened it, in addition to their normal pull.
Seeing who won
Once claimed, the pack's page shows the winning card and the username of the collector who took it.
A word of caution
A pack nearing its last few units makes the Last Card more likely for the next buyer — but "nearly sold out" is not "guaranteed to be you". Other people are buying at the same time.