You can think of this graph as a set of three parallel universes with time flowing from left to right, so that A is the beginning of recorded history. (The arrow represents the "follows" or "is subsequent to" relationship, so you might say that "B follows A".)

To see an ASCII version of this graph, type
git rebase --help

If you start from E, the history you'll see is A, B, C, D, E.

If you start from H, the history you'll see is A, B, F, G, H.

If you start from K, the history you'll see is A, B, C, I, J, K.

But the really important thing about this is that no matter which node you start with, some parts of the graph will be unreachable to you. That sounds kind of pessimistic, so I'll turn it around: Depending on where you start, you can reach parts of the graph that you couldn't get to otherwise.

This may not sound like much, but stay with me for a little while longer—it's huge.