what is RCB

it is always appearing in the terms of HYIP programs

RCB = referral cash back

It means, in your example, someone is offering to give you 150% of the referral fee they receive, if you join and deposit using their referral link.
In theory it would work this way: if the referrer gets $.50 referral commission from a deposit you make, s/he would send $.75 back to you.