On Snapchat, "RS" typically stands for "Reply Snap." It is often used when someone wants you to respond to their snap with another snap, rather than just leaving it on read. This is commonly seen in streaks, where users exchange snaps daily to keep their streak going. In some cases, "RS" might also stand for "Resend Snapchat planets," meaning the sender is asking you to send a snap again if they missed it. Context is key, so the meaning may vary depending on the conversation.