This happens when you buy or sell with a market order, but the purchase or sale can't be fully executed at the current market price.
For example, you've got 100,000 X coins. Their current price is $10. You start selling with a market order, but there aren’t enough buyers at $10 to scoop up your entire stack of 100k coins. For instance, buyers wanting to grab the coin at $10 have only put up a buy volume of 10,000 coins. So, at $10, you can only offload 10,000 coins, and then the price is gonna tank down the order book (the exchange will start moving down the buyer's book and lowering the coin's price in search of new buyers at a cheaper price).
Then the price might drop to $9, for example, where there are buyers for 20,000 coins. You might sell them another 20,000 of yours, but at the price of $9.
Then the price will drop to $5, where there are buyers for 50,000 coins. And you sold another 50,000 coins at the price of $5.
So the price will keep dropping and dropping until you sell all 100,000 of your coins. However, you won't sell them for the $10 you initially planned, but a bit cheaper. This is the danger of selling low-liquidity shitcoins with a market order in large volume. You're crashing the price yourself. On the candlestick chart, this will look like a 'dump'.
Buying with a market order is exactly the same, but in reverse. If you want to buy 100,000 coins at $10, but there are no sellers at that price with that volume in the order book, the exchange will start moving up the order book higher and higher (the price will start to rise, and the exchange will push up through the sellers to find new ones at higher prices).
For example, at $10 the exchange will buy you 20,000 coins, and there are no more sellers at that price in the order book. So it will start looking for sellers at higher prices and buy you more coins at $10.5. If there's still not enough volume at that price, it will keep moving higher and buy you some coins at $11. And it will keep climbing the order book, buying your coins at higher prices. On the candlestick chart, this will look like a 'pump'. Thus, you'll already be taking losses at the moment of purchase, since you planned to buy 100,000 coins at $10, but you'll end up buying them at a higher price, meaning you'll pay much more than you initially planned. 🤷
In the end, this can turn into a real nightmare if you decide to buy or sell a large volume of low-liquidity junk with a market order. But what about buying with a limit order? Yes, with a limit order you buy or sell coins strictly at a fixed price. However, if the volume of coins is large, they will take a long time to sell or buy with a limit order.
So just don't mess with low-liquidity garbage. Avoid such coins. Never buy them for tens or even hundreds of thousands of dollars.
With normal coins like BTC, SOL, ETH, BNB, TRX, and others, this will never happen. The trading volumes are in the hundreds of millions of dollars, and even your market order for tens of thousands of dollars to buy or sell will be easily absorbed at a favorable price without slippage.