A bindstone is a magical structure to which your soul is bound to, so in a case of death you will respawn there. There are various bindstones all around Agon, most of them in NPC cities and player holdings.
You can be bound to only one bindstone at any time. Binding to a bindstone happens by looking at it and holding F when close enough.
Player holding bindstones have a few restrictions. Only clan members can bind to the bindstone that their clan owns and the bind slots are not unlimited, though it can be increased by building certain holding structures.