Capitalism is, fundamentally, about the underlying property relations at the base of society. Claims on ownership granting special rules. So kind of like how in feudalism - the King got a bunch of special rules that only applied to him (sometimes from God), and later some of those would extend to the nobility and aristocracy, but not to the serfs and peasants - or how in ancient Rome, masters got one set of rules and slaves got a different set of rules - well Marxists make the argument that we've still got a two-tiered ruleset in place today - where one group of people (or CLASS of people) have to live and play by a basic set of rules, but another, smaller group (or CLASS) of people get some bonus special rules that help them out extra and give them bonus power and authority that members of the much larger group don't have the same access to or ability to wield. Even moreso, Marxists make the argument that these bonus rules are actually detrimental and damaging to the larger group, in order to amplify the benefit provided to the smaller group.
So let's talk about these two groups. The first group is probably the one that you and I are a part of, as is the overwhelming majority of humanity. This group is a CLASS of people that acquire and grow their wealth - that is to say, make their money, earn their income, etc - primarily through doing work -ie/ labour. This can be a lot of different things: flipping burgers, writing code, building houses, transporting goods, etc. Lots of different things. But how they all get paid is largely the same. They perform this task, over a certain amount of time, and they receive money from the person or people who owns the business at a fixed rate of pay, multiplied by the amount of time that they spent working. So you make X dollars per hour (called a wage), or you make Y dollars per month or per year (called a salary), or you get paid Z dollars for doing a specific job that will take a specific amount of time.
Now there's a lot of interesting characteristics to talk about with this relationship - but one of the obvious ones is the mathematical limit to wealth growth through labour. Karl Marx calls this group the PROLETARIAT.
If you are getting paid at X dollars an hour, then there's only so many hours in a day that you can work, (and lets face it, you need to sleep to some extent, and there's likely transit and other life obligations involved in there too) and there is a clear upper limit to how quickly you can grow your wealth. Yes, if you have the fortune of being born into a very privileged position, you might be able to negotiate a higher X dollars per hour rate, but it is still a fixed rate, and it still is capped off by how many hours you can physically perform work over the course of a day (or a week, or a year). So even if you are a super skilled, super hard worker who negotiated a good contract, you can still only grow your wealth arithmetically - in direct proportion to the time you put in. But overwhelmingly, that's not how fortunes are made.
Now let's talk about that other group in society - the much smaller and much more powerful one - the one with all the fortunes - the one that really gets to make use of those bonus rules I mentioned. So remember, the PROLETARIAT primarily makes their money from doing work - that's the defining characteristic of that class. Well this group, who Marx calls the BOURGEOISIE, grows their wealth, makes their money in a very different way. Their wealth does not primarily come from doing work - their money comes primarily from ownership claims. They make their money simply by owning things.
Rich people sell you a story about working hard for their money - for the most part, that's a myth - most of their money is made via ownership. In the old days is was the certificate they had that said 'this factory belongs to me (or me and my business partner), and in more modern times, it's divvied up a little differently with things like stocks and bonds - people with differing amounts of equity and portions of the total ownership claim. But the money they make from the ownership claim - that money is made while they sleep and play golf. That money is the money made by the workers, that they only pay a fractional portion back to the people who did the labour to make the money in the first place (salaries, wages, etc). The owners pocket the rest, and the mechanics of how this system works is not the standard of history, but something that has come into place only in the past few hundred years.
