The arrows are pointing in the vertices of the prism. Vertices are the corners of an object which faces are the 2 dimensional sides of an object and the edges are the lines link 2 vertices. Vertex classically defines a corner or a point where lines met. Vertex is likewise from time to time used to designate the 'top' or high point of to some degree. Another example such as the vertex of an isosceles triangle which is the 'top' angle conflicting its base.
