As henesua said an aabb node contains data about the material of the tile (the faces section of the aabb node to be more secific). It's used to play the proper sound (like stone, wood, dirt) and defines where the player can and cannot walk.
Then there is a aabb section in the aabb node. AABB = Axis Aligned Bounding Box (Tree). Normally used to detect overlap of two objects in 3D space, i.e. collision detection.
Like the name suggests it's a couple bounding boxes stored in a tree data structure. If I remember correctly the format for NWN (the aabb section of the aabb node) is: box minimum (x, y, z), box maximum (x, y, z) and the face the box belongs to (-1 if the tree node is not a leaf). An exporter has to generate it from the mesh of the aabb node.