"RS” on Snapchat typically stands for Roundsnap, Rapid Snap, or Restart Streak — depending on context — but most commonly, it means
RS = Streak Reply/Snap — it's a way to let someone know you’re sending back a Snap just to keep your Snapstreak alive.
Often, people will just send a photo with “RS” to maintain their daily Snapstreak with friends.
If you'd like, I can go over other Snapchat abbreviations as well (like S, SB, NRS...). Just let me know