Rep power is the amount of power you have to alter another person' reputation on the board.
If you appreciate someone's posting and want to add to their reputation you click on the little scales symbol in the above bar (on the right). When you click 'I approve' and then click 'Add to reputation' your rep power (in your case 174) would be added to their reputation total. In the same way, if you don't like what someone is saying or feel they're wrong you can take away their reputation by clicking 'I disapprove' and it would remove 174 points from their reputation.
More about reputation can be found in the
AIUK Support and Knowledge Base - do a search on reputation.