Thalia's Pine Tree is a pine tree which marks the Percy Jackson.
Five years before Percy got to Camp Half-Blood, Thalia Grace], Annabeth Chase, Luke Castellan, and Grover Underwood were traveling to Camp Half-Blood. Thalia is the demigod daughter of Zeus; Luke is the son of Hermes; and Annabeth is the daughter of Athena.
Thalia's birth was considered an outrage considering the fact that the Big Three (Zeus, Poseidon, and Hades) made an oath to never sire demigod children. Since Zeus broke this oath, Hades was furious and sent a whole army of monsters from the Underworld, including the Furies, after them. Grover told Percy in The Lightning Thief that he made a couple of wrong turns (one of the worst towards a Cyclopes' lair in Brooklyn) and reached Camp Half-Blood too late. The monsters were only a few yards behind them. But before they could reach the camp's safety (inside the camp's magical borders at the entrance), the monsters attacked.
Thalia told Luke Castellan, Annabeth, and Grover to run while she confronted the monsters. At Thalia's request, the others ran for the borders of Camp Half-Blood. During the battle, Thalia was mortally wounded. As she lay dying, Zeus took pity on his daughter and turned her into a pine tree to preserve her soul. From that day on, her pine tree represented the camp's border, and gave the hill its name: Half-Blood Hill. It fended off monsters and mortals with the magical border around the camp.
The only way for a monster or mortal to get in is to be summoned inside the the camp or to be invited into the camp as shown in The Sea of Monsters when Annabeth allowed Tyson, Percy's cyclops half brother, into the camp just so he could help Percy defeat the bronze Colchis bulls that were invading Camp Half-Blood and also in The Last Olympian where Percy is shown a vision in which May Castellan is invited to take the job of the Oracle.
Percy, Annabeth, and Tyson run off to go looking for the only cure for Thalia's tree, the Golden Fleece, after it had been poisoned by Luke. Coincidentally, Grover found it while looking for the god of the wild, Pan. After their adventure in the Sea of Monsters, they put the Golden Fleece on Thalia's tree. Immediately the tree begins to heal and expel the poison, but it worked too well. Several days (or weeks) later, while Annabeth was on watch (because there was no guardian yet), Thalia was expelled from the tree, having only aged at the rate of the tree. However this was Kronos' plan all along to give him another chance at controlling the prophecy.