The bottom right arrow represents a positron (the anti-electron) - in this convention, all anti-particles are drawn with a reversed arrow, so while the electron has an arrow that points forward in time, the positron has an arrow that points backwards in time. You might ask: why so? Simply put, an antiparticle is a particle travelling forward in time! We'll have more on that later; for the time being, suffice it to say that this is the convention we're going to adopt.
You'll notice two dotted lines at the top, which corresponds to two identical photons - you might notice that there are no arrows at all. Want to fashion a guess? It's because a photon is its own anti-particle, and hence no arrow is needed to distinguish between a photon and an anti-photon at all. Hey wait a minute! Are you saying that photons can actually be travelling forward and backwards in time? Perhaps, for light itself defines the very limits of time travel, and maybe only a photon can experience true time travel without being altered itself.
So how do I read this diagram? Easy, if we use the conventional wisdom of our real world, we say that an electron and a positron travel towards one another, collide, annihilate, and their mass energy is converted into electromagnetic radiation in the form of two photons that travel away from one another.
But in modern Physics, there is an alternative viewpoint: an electron moves from the left to the right, and at one moment in time, emits two photons. The emitting of these two photons causes the electron to change its momentum in time, and causes it to move backwards in time, becoming a positron that goes on towards the right, but backwards in time.
Well, that's all for this post - till when I'm feeling awake again!