The only thing the Curators can do is add/remove addresses from the whitelist. The whitelist is the list of addresses which are able to recieve ETH from the DAO. The Curators exist purely to prevent a 51% attack.
The 51% attack is when a malicious majority tries to put forth a proposal to send ALL of the DAO's ETH to an address "they" (the malicious majority) control. Active DAO token holders will see what is happening and vote "no" on the proposal. Since the (in this case malicious) majority will still win with a "yes" vote. The active minority will then vote to split the DAO which they cannot be stopped by the majority. HOWEVER, the INACTIVE DAO Token Holders(DTH) who aren't participating in voting and simply holding/trading tokens (perhaps in an exchange such as yours) will have no idea what is happening. Therefore, the 51% attacker would get THEIR ETH + SUM of ETH of all inactive DTHs.
The Curators stand in the way of this because they will see this attack and NOT add the 51%'s ETH address to the whitelist, thus effectively blocking the outcome of their proposal.
The Curators CANNOT interfere in any way with the proposals themselves (ie rejecting or approving them)
Also, with the Curators in place in the above scenario it would be uneccesary for the minority to split the DAO as the malicious majority's ETh address would not be able to recieve any ETH from the DAO.
This also means that there is a certain level of trust in the Curator. This is exactly why it is currently set up as a multi-sig contract populated by prominent members in the ETH/Crypto community. BUT, the option still remains to replace them at any time.
Does that help to explain your question?
The DAO