An electron encountered a positron: they are inexorably drawn towards each other, but their combination is intended to be fatal, and from their union remains only a photon, traveling... and traveling... and traveling... and occasionally it disappears, it decomposes into an electron and a positron. These, however, are futile images, as long as they do not possess enough energy to go away, to far away from each other.
Until the next waltz.
The image (via Quantum Diaries) represent a Feynman's diagram about the electron-positron annihilation and a next pair production. In the middle of the diagram, there is the auto energy of the photon, and it is named loop, and the diagram, loop diagram.
Read also: Let’s draw Feynman diagrams! | Feynman diagrams (pdf)