A pump and dump is a scam very common in traditional finance, where scammers make hyperbolic or greatly exaggerated claims about an asset in order to get others to buy into it. The scammer at this point would already own a lot of such asset, and once the price increases due to their marketing / scamming efforts, they sell their assets, resulting in a dramatic drop in price of the asset. As the name suggests, this is a rapid pump in asset prices, and then a rapid dump in asset prices.