The Runestones push out a magical shield that cannot be passed through except for a few hidden spots as well as the main gate of Athel Loren located around Quel'Lania. The runestones themselves emit a powerful noise which causes though's trying to go through their area to become extremely disoriented to the point that if they do manage to get through the length of the shield they are either easy target for the Elven defenders, or left mentally devoid.
The Runestones do not stretch across the entirety of the forest, but they are designed to surroud the most densly populated areas of the forest so that in an attack the populated areas will be safe. This strategy means that for the areas outside of this including much of the villages of the forest they are still open to attack. For the defence of these areas the localized defenders, and the Sentinels still stand as the main line of defence, but the Runstones have pushed back the larger quanitity of attacks.
The construction of the Runestones was a religious effort for the Lothlorian Elves, and for the Sentinels/Sindar Elves this was the culmination of all the work the forest needed to finally protect itself. Following the contruction of the Sunwell, and Silvermoon city the Elves begin to expand beyond the natural bounderies of Silvermoon, and the region around it, and thus with this expansion came the growth in deaths amongst these expansions from the nearbye Trolloc who still entered the forest basically at will. In order to change this the easily accesible areas of the forest were laced with Runestones that made entry through them extremely painful except for through small entry points. Having entry points meant that the Sentinels, and the rest of the Lothlorian army could more easily focus on the attackers and not have to defend a vast forest that spread them too thin. As these runestones came to be more numerous the expansion of the kingdom came to coincide with the location of the Runestones, and thus in areas where they were heavily concentrated you would very likely find a large settlement.
Civil War in Lothlorian
Main Article : Lothlorian Civil War
During the events of the Lothlorian Civil War the runestones were menipulated by the forces of Quel'Thalas in an attempt to make the Runestones turn insular and destroy anyone inside them that used Magi. This effort culminated at the Battle of Runestone Mal'dor where in the Sindar Elves were defeated and the runestones reverted to their true purpose.
Post Civil War
Following the events of the civil war the runestones have returned to their true purpose of defence, and their existence has allowed the Lothlorian Elves to heavily concentrate their forces at the gateways into the forest as opposed to having to defend the entire forest border. This concentration has meant that the main gate to the inner forest in Skryion Gate is heavily defended by over 30,000 of the most heavily trained troops in the KIngdom of Lothlorian.