Pump-and-dump is a scheme aimed at artificially inflating the value of a coin or token. The stock is talked-up by people who make misleading or false claims. This hype artificially inflates the price, at which point the pumper sells (dumps) their supply, usually at a significant profit. Basically, talk up the value and then dump it when the price rises. This practice is illegal in many countries based on securities law and can lead to heavy fines.
A pump and dump scheme explained in one minute.