In the simplest financial sense of the word, "capital" is what you own.
Money, houses, cars, furniture, businesses, investments – the list goes on.
The problem with living in a capitalist system is that we are like fish in water – we don't notice it because it's everywhere. We can't imagine anything different.
The paradox is that, like racism, sexism, ageism etc, capitalism is oppressive and discriminatory. People are judged and treated differently based on how much capital they own.
Capital brings with it power and privilege. How much capital you own (or even just have access to in certrain circumstances) dictates your status and influence in society.
The less you own, the harder it is lo live in the world.
That is, of course, if you continue to buy into the capitalist paradigm. More and more people are choosing lifestyles that can be sustained with smaller amounts of capital.
Nevertheless, "capital-ism" is our biggest oppressor. It creates most, if not all, of our social problems – crime and violence; poor mental and physical health; lack of access to education and employment opportunities; and, of course, poverty.
Until we address capital-ism for what it is, not just a financial system but an unrecognised, systemic regime of power and privilege differentiation, we will never solve other social inequities.