DAO tokens represent your relative stake in the DAO. They give you voting rights and are the mechanism via which rewards are paid. You can redeem your relative portion of the ETH held by the DAO at any time by splitting your tokens from the rest of the DAO. The account holding these tokens will continue to receive reward tokens from any proposals you backed in perpetuity. However, this account will not receive rewards for proposals backed by the DAO after your split.
The other way to retrieve your value is to sell your DAO tokens on the open market, via this options you will receive the market value of the DAO tokens (which should, in theory, be slightly higher than their relative ETH value), however you will not receive ongoing rewards from the projects which you backed.