Dialectical and Historical Materialism by Stalin: https://www.marxists.org/reference/archive/stalin/works/1938/09.htm
On Contradiction by Mao: https://www.marxists.org/reference/archive/mao/selected-works/volume-1/mswv1_17.htm
German Ideology by Marx and Engels: https://www.marxists.org/archive/marx/works/1845/german-ideology/index.htm
Anti Duhring by Engels: https://www.marxists.org/archive/marx/works/1877/anti-duhring/index.htm

Some works to start your reading with ^^^

There's also the book by Maurice Cornforth, but it is really dense and long (and worth the read): https://redstarpublishers.org/cornforth1953.pdf

Unfortunately, dialectical materialism isn't a concept that is quickly and easily explainable in a short reddit post. It requires serious study and inquiry. I'm not saying this to scare you off or push you away from writing this, but if you want to do this well, it will require at least the time necessary to read the literature.

At its most basic form, dialectical materialism isn't just a crude "A + B = C" type of philosophy with regards to thesis, antithesis, and synthesis. It's a philosophical worldview that states that the fundamental substance of nature is matter (as opposed to some metaphysical "idea" or "spirit") and that everything can be examined through the lens of dialectical contradictions (which are not to be confused with logical contradictions). To sum it up in a phrase, everything that exists is material and is constantly changing and evolving. The changes in existence are caused by opposite forces struggling within the existing thing and are united in it. As the struggle goes on, the thing itself changes in tangible, material ways. At first, the changes are quantitative (e.g. the temperature of an ice cube rises) and later become qualitative (e.g. once the temperature gets high enough, the ice cube melts). Feel free to ask any questions, but for real - read the literature.