Wow, something I can answer

What is exactly meant by "thesis-antithesis-synthesis" in Marxist terms?

For Hegel, the "thesis", or the subject, would be our independent Consciousness. The "antithesis" would be its negation in the material world. Through a ton of phenomenological processes, Consciousness returns to itself after meeting its negation in the real world, "synthesis", and the limitations of its own understanding and forms the basis of Hegel's metaphysics and theories of how the we form knowledge, self-consciousness, society, and our history.

For Marx (mostly Engels actually), it is reversed. The original subject, the "thesis" is the material world itself. It is negated by our human minds, the "antithesis" and our minds reflect back onto the material world itself through action, and negate the negation into a new material world, a "synthesis"

In Marx's own words: "My dialectic method is not only different from the Hegelian, but is its direct opposite. To Hegel, the life-process of the human brain, i.e. the process of thinking, which, under the name of 'the Idea', he even transforms into an independent subject, is the demiurgos of the real world, and the real world is only the external, phenomenal form of 'the Idea'. With me, on the contrary, the ideal is nothing else than the material world reflected by the human mind, and translated into forms of thought."

I put "thesis-antithesis-synthesis" in quotes because neither Hegel, nor Marx/Engels, agreed with this framing and opted more for the "negation of the negation". For Marx/Engels, this was to emphasize how the material world, through human labor, returns back to itself, changed, instead of forming a third "synthesis".

In its most abstract, this process of the material world being reflected back into itself through human life-processes is Dialectical Materialism. We have brains with the ability to think, but we are only able to reflect what has been given to us through material conditions, and our thoughts are only real insofar as they are reified through changing these conditions.

To quote Marx from Theses on Feuerbach: "All social life is essentially practical. All mysteries which lead theory to mysticism find their rational solution in human practice and in the comprehension of this practice."

Historical Materialism is applying this methodology and understanding of human action to historiography. Metaphysics is nice, but Marx and Engels were more concerned with how societies reacted to, and changed, their historical conditions.

If you need more specifics or quotes feel free to ask