The “greater fool theory”:

It states that investors can make money out of overpriced assets by selling to another person who will pay a still higher price, in hopes of finding a “greater fool” to purchase at an even higher price. This also depends on the belief that the market may keep inflating an asset’s value through speculation and hype regardless of not being backed by intrinsic value.
Here is a more expanded explanation:
How it works:
Speculation and Hype:
The theory functions in markets where prices are driven by speculation and hype, frequently resulting in a bubble.
Buying Overvalued Assets:
Investors acquire assets that are priced above their intrinsic or fundamental value with the expectation of profiting from the market’s further rise.
Finding a “Greater Fool”:
The underlying belief is that there will be another investor (the “greater fool”) who will pay a higher price, allowing the initial investor to come out of the trade with a profit.
Bubble Burst:
This process can go on until the market becomes unsustainable, and the bubble collapses, triggering a steep price drop, quite often leaving the final investors to buy the overpriced asset to become the “greater fools”.
Examples:
Cryptocurrencies:
The higher-than-usual price appreciation of cryptocurrencies during 2020-21 was partly driven by the greater fool theory, with investors wagering on higher future prices irrespective of underlying fundamentals.