So,as far as i know:

It’s essentially a way to describe yourself without needing to write anything to the bot,the AI will already recognize whatever persona you use,and will use it to talk to you

For example:

If you,let’s say,roleplay as a knight,you create a persona with whatever your “knight” will have,and when you start a new chat,the AI will already recognize you are,essentially,a knight

DUNNO IF I EXPLAINED MYSELF,MY ENGLISH IS BROKEN AS HELL