Prop firms require you to pay for the opportunity to trade on their demo account under very strict rules & if you are profitable without breaking their many rules, they will reward you with some money which they call a "payout".
However, if you fail to be profitable or if you break their rules while making profit, they will close your account & ask you to pay for another challenge (another opportunity to start over).
The prop firms make their money from people who keep purchasing & re-purchasing their challenges after failing or after having their accounts closed because they broke a rule.
Statistics show that around 95% of traders will fail a prop firm challenge either due to not meeting their profit target or due to breaking a rule.
There are several rules put in place by prop firms such as maximum daily loss limit, restriction from trading during certain news releases, using EAs, taking on too much risk, etc.
If you are able to pass the prop firm challenge, they allow you proceed to the next phase where you will be handed another demo account (aka funded account) but this time you do not have to pay for it. Instead, any profits you make on this funded account will be shared/split with the prop firm.
So, if the prop firm has an 80/20 split, you will take 80% of the profits while the prop firm keeps 20%.
Even after you have progressed to the stage of being handed a funded account, you still need to abide by the prop firm's rules or it will be taken from you.