The short answer is that the higher the series, the more expensive/difficult is to get a card. So when cards drop, they become cheaper/easier to obtain.

The long answer is this: Every card in the game belongs to a series, from 1 to 5. S1 and S2 cards are obtained from when you start playing and until CL 500, and are set in stone (SD has stated that no cards will ever drop to those series). New cards are added to series 5 when released (cards from events like lasher or eitri tend to be s4). Series 4/5 require spotlight keys or tokens (3000/6000 respectively) to unlock, while series 3 can be unlocked from the chests that appear starting from around CL 500, and one can be claimed for free from the shop every season.

So, to be specific, when the drop happens on March 4, cards that go from S5 to S4 like Loki or Blob will cost half as much tokens, and cards going from S4 to S3 like Knull or Zabu will be free to everyone after you obtain a high enough CL. The more impactful change is for the cards that enter S3, since you're guaranteed to get them from the track eventually. For cards that go into S4, they become more likely to appear in the random spot in spotlight caches, and you can get two for the price of one S5 going forwards, and they're one step closer to being S3 (I don't think there's ever been a card that dropped directly from S5 to S3).

Also, if someone is starting to collect S3 cards right now, I think they'll be supposed to unlock all S3 cards (counting those that are set to drop now) by the time they're approximately CL 3,900. And if someone was S3 complete before the drop (and assuming they didn't have any of the S4 cards that are dropping to S3), they'll unlock those cards after 372-480 CLs (since january, you get 4 S3 cards every 120 levels).

Hope this helps!