Zero Sum Game is a concept from Game Theory and Economic Theory, where the gains or losses of one participant are exactly balanced by the losses or gains by other participants.

This means that money you make from trading crypto is the result of other people having lost money from trading crypto.

For example, you buy BTC when it is $5000. It then goes up to $20,000 like it did in December 2017, and you sell because you're happy with 4x (!!!). Someone else buys that $20,000 BTC from you, and eats the loss as it plummets into the ground (bottoming out at $3k a year later).

It's not always that direct, for example you buy at $5000, you sell at $10,000 and it keeps going up so the person who purchased from you can still make money. But someone out there in the market is losing money to support it. These could be people shorting the market, buying options, or what have you.

In short, money is not being created in the markets. BTC going up 4x is not adding more money to the world, it's just the result of money having left the stock market, gold, bonds, or cash reserves.