Rep power is a measure of forum contribution that is determined through some mysterious computer-driven calculation. To my understanding, one rep-power is awarded to a member for each 1000 posts he/she makes.
Other factors affect the Rep Power number as well, such as time since first joining SailNet and the amount of Rep Points a member has accumulated. Rep Points are displayed in the bottom of the User CP screen (upper left hand top menu bar).
These points are either increased or decreased, based upon the negative or positive "hits" other posters provide. To do this, click on the little scale icon

in the upper right hand corner of the blue bar on each post.
EDIT:
Strange that the sailnet site went down again - my post was posted before I had a chance to finish.