Experience points indicate how many matches you have played. New players begin with zero experience points, which then increases for each match completed (for rated and unrated matches only). The experience points for GridGammon guests are capped at 100, guests with more than 100 experience points are shown as having experience of 100.
Experience points are determined by the match length, not the final score. When you complete a three point match, both players' experience points will increase by three, even if the final score was 4-1.
Experience points are used in the GridGammon ratings calculation, players with less than 400 experience points will see larger changes in their ratings.