Rp
Rp has 3 meanings
Reza Pahlavi
RP are the initials for Reza Pahlavi, the oldest son of Mohammad Reza Pahlavi (the last Shah of Iran), making him the last crown prince. People on social media often refer to him as RP instead of "your majesty" at Reza's request.
Before the Iranian Revolution of 1979, RP was set to be the heir to the throne of the Imperial State of Iran. He is now a proponent of democracy, hence the request to be called "RP." Unless you follow Iranian affairs, you probably are not familiar with RP or the initials' use on social media, especially Persian social media.
Example
The last I saw on Twitter was that RP is urging more harmony between the US and Iran
Interesting
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Rp is short for "roleplay" (which may also be spelled "role play" or "role-play"). People often use this abbreviation when playing RPGs, but anyone engaging in a bit of make-believe may also use it.
When roleplaying, a person mimics the personality and behavior of a (usually) made-up character. For example, DnD players often roleplay as fighters, wizards, rogues, and other fantasy characters.
However, anyone can engage in roleplay, and roleplay does not need to involve fantasy characters or even RPGs. For example, therapists often engage their clients in therapeutic roleplay, and some couples (and throuples) enjoy roleplaying to spice up their love lives.