To understand it a assume there is an electron entering and leaving at the tail of the tadpole (Hartree diagram) and an electron entering and leaving at the edges of the Fock diagram.
If I were to guess the meaning of the diagrams I would say:
Hartree: An electron interacts with another electron, described by the circle, which first appears with some momentum and the suddenly disappear giving the momentum back to the first electron, and in the end no real change happened.
Fock: An electron enters, interacts with itself, (I do not understand the result of this self interacting) and then moves on, and in the end no real change happened.
I don't find it to suspicious that no real change is happening, this is just the way a particle is propagating.
I have no clue if these explanations are even close to the correct interpretation and would appreciate any help and input!