Gear score is numerical value based on the statistics of a player's gear. In addition to item level (ilvl), the item's actual statistics or abilities may or may not be included for calculation, as determined by specific formulas or addons.
Most commonly, gear scores are a metric used to evaluate the relative strength of a player's equipment as a way of determining if that player meets a given threshold to join in certain raids or groups going through an instance. It should be noted that a given gear score, no matter which formula is used, does not necessarily mean that other players can not demand higher scores from others before joining a raid or group. - Stolen from WoWWiki