Though not fully revealed until much later into the run, The White Lotus are arguably some of the most important characters in the Avatar universe. And if you paid attention, they were teased in several episodes before their reveal.
The order itself was formed long before Aang's time. They were a group of scholars and wisemen who wanted to play Pai Sho with each other, regardless of loyalty or nation they came from. They believed in going "beyond borders" to find beauty, truth, and enlightenment.
One such member was Iroh, who was the one who brought forth the rallying call for the order to rise out of the shadows and help save the world. Thus they gathered at Ba Sing Se to liberate it during the Sozen's Comet event.
After revealing themselves to the world, the White Lotus worked with Avatar Aang to help bring balance back to the world. They helped him build Republic City, and helped find the new Avatar, Korra, after Aang's passing. They then safeguarded and trained her. And even fought beside her many times throughout the series.
The White Lotus still exists today, and as long as their philiosophies are still believed in, they will endure.