It's not a new idea in the general sense so you can find a few different ways it might be defined.

When it comes to Marx, it's about what he took (and often flipped on its head) from Hegel, where Hegel was the first nerd to get really into dialectics when it came to analyzing society.

A very short summary of dialectics wrt Hegel and Marx is that it is an often-useful tool by which you can frame a thing is actually by the states through which it may transition, that a thing is better described by what it is and how it can become not that, and then by looking at the reasons this would happen, produce salient contradictions that explain the thing itself. This sounds confusing because it's abstract, but it's not really that foreign to most people nowadays.

Example: there is a fly buzzing around your room. We can say that the fly is alive. Eventually, it will die, decompose, and become earth. We have the state of the thing (alive), another state of the thing (dead/dirt), and say that a fly may be better understood by knowing the processes that move the fly to or away from either state. And, most importantly, a hard subscriber to dialectics would say that our understanding of the fly should be about these processes in opposition, not a list of descriptions of the fly in the "alive" state. Metabolism keeps the fly alive. An end to metabolism, such as being exposed to the cold for a long period, leads to its death. It must constantly eat food to live. If it stops eating food, it dies. We can keep enumerating things to come up with insights that we believe are important until we feel satisfied that we have understood and distinguished the fly from other things via this process.

This example is very similar to Hegel's formation, where we understand the world through DEEP THOUGHTS and, in fact, reality relative to human interactions is derived from them and can only be understood via the correct extraction of those thoughts. Marx liked the processes and contradiction part, but disliked the ideas --> reality part, flipping it around: material forces stand in opposition, they are the processes, the transitions, and instead, ideas are the product of material things. A simplistic example is that Romans didn't have an opinion on motorcycles, but there are deeper and subtler implications that Marx related to economic production.

This is where simple fights about idealism come from as well, with Marxists appealing to economic forces determining the population's ideas and socioeconomic transitions and crapping on people who think that it's mostly about convincing people that the right ideas are true - and if we just had that, we'd win.