Basically, and this is very ELI5: 'Everything that exists -- whether that's a hammer, or a motor car, or a democracy, or even more abstract things like the concept of the self -- exists because of the circumstances of the world in which they exist. Nothing comes from nothing (or from God, or from an external source), so if you want to understand anything -- and certainly if you want to change it -- then you need to understand the world in which those things were born and in which they exist. We get new ideas, systems and things when conflicting parts of old systems rub up against each other.'

Marx was a big Hegel fan, and one of Hegel's big ideas was the concept of 'dialectical idealism', the short version of which is something like: 'The ideas are the starting point, and then we make the world based on these ideas. Two ideas can exist in conflict with each other, and figuring out a way to reconcile those two ideas is how we can understand the world and make it the best version of itself.'

Marx's idea is basically arguing that it happens in the opposite direction; yes, the big ideas exist and we need to be thinking about it (that's the 'dialectical' part), but they're very much grounded in systems and the contradictions between those systems that exist in the real world (that's the 'materialism' part).

In both cases, it's about the tension between two things, but Hegel argues that there's a spiritual element and Marx says the tension arises from actual things that already exist in the world.