The positron is the electron going back in time. This is what the arrow means. But you have to remember that in physics, cause and effect are not by the direction of the arrows, but by the direction of entropy increase, which is up in the diagram regardless of which way the particles go relative to their own proper time.

The idea that positrons are electrons going back in time is due to Ernest Stueckelberg, but it was misattributed to Feynman, who got it indirectly from Stueckelberg through Wheeler. The idea is correct, and it solves the problem of creating a relativistic _particle_ formalism. But it requires that the particle trajectories are not causal, they either produce an S-matrix, or correlation functions. The notion of causality is only for a higher level description.