According to Marx capital is the wealth (including not only money, but—more essentially—also land, buildings, machinery and hired labor) that is devoted to the production of more wealth. The capitalist hires workers, who make practical use of that existing capital (factories, machines, raw materials, etc.) to create additional value which then automatically belongs to the capitalist. This retained surplus value becomes part of the expanded capital that the capitalist then owns.
If an ordinary person, a worker, has some money saved up, or still available at the end of the month, that is not “capital”! (At least in the Marxist sense.) The reason is that the worker has no way of making use of that money to create more value, more capital. The worker does not own a factory, machinery, the necessary raw materials, etc., that would allow him to do that. On the other hand, even if a capitalist is deeply in debt, and needs to borrow more money to keep functioning, if he already owns a factory, machinery, raw materials, and purchases labor power (i.e., hires workers) he already has a tremendous amount of capital, and is in a position to make use of it to generate additional capital.
Money can be used as capital (e.g., to build factories, buy machinery and raw materials, hire workers, etc.). Money which is actually used for such purposes is called money capital. But if money, whether a small amount or a large amount, is not used for such purposes, it is not capital. Marxist definition of "Capital" from The Dictionary of Revolutionary Marxism
I want to highlight one bit of their definition:
If an ordinary person, a worker, has some money saved up, or still available at the end of the month, that is not “capital”! (At least in the Marxist sense.) The reason is that the worker has no way of making use of that money to create more value, more capital. The worker does not own a factory, machinery, the necessary raw materials, etc., that would allow him to do that.
While that may have been true in 1848 it is not true today.
Today, any person can participate in create more value and more capital by investing in the stock market or bond market via mutual funds. They can invest in their brain via formal education or through the intelligent use of the internet. They can start a business and sell on eBay, Etsy, farmer-markets or push-carts. They can give back-rubs or clean houses or do yard-work.
The keystone that locks the archway of Marxist thought together is a lie. The premise at the center of Marxist thought is as goofy as a seven-sided square.
Peeling the reality just a tiny bit, the "ordinary person" owns stock in the form of insurance policies and retirement plans. There is not a safe filled with stacks of $100 bills waiting for them to make a claim on their auto insurance.
The paycheck that will land in an "ordinary person's" account is currently racing around the world in the 24 hour bond market. That dollar does not go to sleep in NYC. Rather, it races to Tokyo and then to Europe before the sun rises again in the United States.
The streets the "ordinary person" drives on were funded, in part, by bonds sold by the state or city. Those bonds were purchased by "surplus value" seeking a safe haven. Without the capitalistic bond market there would not have been money to rent equipment, purchase concrete and steel reinforcement and to pay the workers. The "ordinary person" would be driving on crappy, 3rd World roads to get to work.
Marxists have a very childish view of how the economy works.