REDPOS is just an idea for a new type of DPoS protocol that uses REPUTATION as part of the witness rank. That link is the only paper defining it, for the moment.

For example, this account:

Could be elected as witness, because the Reputation is higher than 60 and the acount age is older than 25 moons.

Now look at this other account:

That account could be a user on a REDPOS chain, but it can't be a witness (no matter how much voting power), because Reputation is lower than 60 and account age too young.

Of course, parameters are arbitrary (any parameter could be used), but the idea is that it acts as a barrier to new accounts without enough REP.

Since the chain is oriented to human communities it doesn't make sense that a newly created account can become a witness of a 4 year old chain. Doesn't make sense on the real world either.