Again, why post if you aren't going to actually help... I don't think he just wanted to know what RSP stands for !!
I'm no expert but, its one of the plugin options... You mostly don't need to mess with it at all, but for some games you need to switch the CPU core style from Recompiler to Interpreter. Personally, i never touch it. Its the "configure RSP plugin" option in the options menu at the top the pj64 browser window.
Not a great insight but hope it helps a little...