A pump is the first half of a pump-and-dump. Price is driven up while enthusiasm is manufactured - posts, calls, screenshots, influencer mentions - so that a larger group buys into the strength. The organisers then sell into that demand.
Pumps are most common in low-float shares and thinly traded tokens, where modest volume moves price a long way. Organising or promoting one is market manipulation and illegal in regulated markets, and paid promotion without disclosure is an offence in many jurisdictions. For the participant, the reliable observation is that if you learned about it from the promotion, you are the demand. See exit-liquidity and shill.
Related: pump-and-dump, exit-liquidity, shill, market-manipulation