Materialism is the understanding that physical matter and energy are the source of ideas. Everything exists whether it is observed or not. Idealism is the approach to reality from the incorrect basis that first ideas exist before the material world. "The world exists because you perceive it." Spiritualism is kinda in the middle. Everything exists because the spirit made it but even if there were no material observers there would still be spirit.

Dialectics is a method of studying why and how things change from one thing into another. Nothing stays the same. All things have internal contradictions within them driving them to become something else. Ever growing imbalance of internal contradictions (quantitative changes) results in a change from one thing into another (qualitative change). The more complex the thing the more contradictions it has.

So you have a starting point called the "thesis" the contradiction is called The "antithesis" and the product of the process called the "synthesis."

Recognizing the main forces at play (Primary contradictions) to find the antithesis is the hard part unless you are analyzing in hindsight. Contradictions are internal. They are attributes of the thing itself that inevitably build up (quantitative) and result in transformation.(qualitative)

A child, [thesis] cannot stay a child due to its genetic and social blueprints, [antithesis] it will become an adult (if it does not die.) [Synthesis]

The synthesis is a new thing that shares some attributes with the thesis but at the same time it is not the thesis and the thesis ceases to exist in the transition. (This can be called "negation" because the thing stops being what it was and the "negation of the negation" is it becoming a new thesis of its own.)

Dialectical Materialism is the understanding that material conditions spawned the human mind and the ideas in the human mind can shape material conditions (praxis) which can in turn shape the human mind in an endless dialectical dance.