Dialects explains that the world is full of contradictions and opposing forces. When these opposing forces come together, they can form a new thing. An easy example is to look at multiple opposing news sources to piece together the entire truth of an event.

Dialectical materialism is Marx's concept that contradictions in material conditions of life are what drive historical change. Primarily that the economic base (economic classes, resources, working conditions) is what shapes the superstructure of society (laws, politics, culture.)