What does it mean here: https://stackoverflow.com/privileges/participate-in-meta when it says your reputation on meta is the same as your reputation on the main site because in my experience this is not the case. Can some explain this to me or is it an error in the documentation?
Not an error. Meta Stack Overflow is run as a fully fledge site, and is the Meta for both Stack Exchange and Stack Overflow. All other per site meta, ie. Meta Super User, is linked to the main site and user accounts are synchronised between the two. All sites have their own meta's, apart from Meta Stack Overflow.