The term refers to someone advertising their own NFT creations or an NFT project they are interested in to get others to buy in. Also known as pumping.
The Long Explanation
The term is widely used in the NFT world to refer to someone actively promoting (sometimes maliciously) an NFT or cryptocurrency, which could be their own creation, encouraging others to invest in the same or a particular NFT project that the person is interested in. This is to increase the value of the assets and their own profits.
Those who do this are often called shills.
Shilling is typically done on social media when people display or share a link to the NFT or NFT project they support or is their own, to make it seen and bought.