Advantage is a multiplier to the amount of victory points earned from completing faction warfare plexes and battlefields. The higher the multiplier, the more victory points are earned. Victory points (Victory Points and Contested %) is the count (number of points) needed for a system to become vulnerable. This is typically represented as the contested value (%). In other words, the higher the advantage for one militia, the faster that militia can change the contested value in fewer plexes.
https://www.aideron.org/fw/map or https://www.eveonline.com/frontlines/gallente to see current advantage % in a given system.
Thanks to Rothar Luke, Chaunnay Solette, Quinn Rhodes, and others for fits and details on sites.