This is my opinion. Not really super well read, but whatever. Take it or leave it.
Historical materialism is the idea that history develops. In other words, that history is a process (but not a determined process necessarily). And that the factors that cause change are present in itself, a succession of historical moments. It implies that the development of history is history passing from one moment to the next, where the impulses of change come from the moments themselves. Passing from one moment to the next. If you analyze history (through a marxist lens), you realize that society is organized around production since the times of the agrarian revolution.
Further, you realize that these developments of historical moments get to a point where the system can no longer be what it was. The repetition of the same mode of development of the means of production, eventually leads to its maximum manifestation, after which the system is no longer stable, which causes society to start to organize production differently, and sets in motion instability that (may) eventually lead to a change in the mode of production. The development of history is that of the development of the means of production, where those that push development forward inherit the historical process, and those that stand in the way get run over.
The bourgeoisie is a parasitic element in the production process. It charges rent and profit for no reason, which makes the production process stall. Further, finance capital is taking over industrial capital, but it only seeks to make profit, not expand the means of production. It is not only a parasitic relationship, it is more like the vampire that sucks the life force out of a company, and leaves it dead to go to the next victim.
So to analyze the development of history, we can't think of the current moment as eternal, we must understand that each moment is ephemeral, and the development of history is always taking place, passing from one moment to the next. If the process we are in continues forever, there will be a moment in which it becomes untenable for whatever reason. If we take any decision that does not "lessen" the impediments to production that capitalism has, then we will not have phase shifted to a higher, less restrictive, mode of development. What this higher, less restrictive, mode of development HAS to be is a form of socialized means of production. Anything that is more restrictive than that is destined to be a repetition of a previously seen mode of production.
To understand this you must first understand Hegel IMO. The way I see it now, which is probably wrong because Hegel is hard to get, is what follows. Basically, where the fuck do ideas (thought) come from? If you think about it, when you see an object, do you see the thing in itself, or do you see the phenomenal appearance (that is, what appears to your senses)? How can you claim to know a thing, if everything it is (it's phenomenal appearance), exists literally only in your head? So what it is, according to you, at least, is developed by you as pure idea. So the way you perceive the outer world is determined by your thought and sense perception. So it's not that thought molds the outer world, but that you can only perceive the outer world as what is "allowed" by the development of your thoughts.
What Marx then says, is that no wait, it is the outer world that molds the gray matter, which is what then determines the things that can be thought and the way the outside world can be perceived as. So that means that your way of perceiving reality is contingent upon your material reality. So that in order to understand the way you think, you must first understand your material reality. Once you have done so, which Marx does via political economy (and using hegelianism (post-hoc) everywhere), then you realize that capitalism is a historical period, and that we are all on a train ride of history, where the main reason for development has been the process of development of the means of production. Which leads to historical materialism, explained above.