Elementary Principles of Philosophy by Georges Politzer is my favorite introduction to Historical and Dialectical Materialism if you're down to reading a book!

If not, the gist of it is that Materialism is a subset of philosophy that believes matter shapes thoughts, rather than idealism where thoughts shape matter. The Dialectical aspect focuses on contradictions both within something and between things, their relations, and trajectories.

An example of the usefulness is when Karl Marx developed it and used it to analyze Capitalism, seeing how it arose from Feudalism, and predicted that because Capitalism has a tendency to centralize and the Proletariat stands at odds with the Bourgeoisie, eventually Socialism will be the next phase, emerging from the conditions laid out by Capitalism via revolution. The class dynamics formed a contradiction, as you cannot have a bourgeoisie without proletarians, as well as vice versa. Additionally, Capitalism contains the means to make Socialism, the internal contradictions.

This was an extreme oversimplification, but that's the bare gist. I recommend the book if you're interested in more!