“Shilling” is when somebody talks up a cryptocurrency project with the goal of creating more demand for a token and increasing its price. Presumably this is done because the “shill” owns tokens and wants the price to increase.

This carries a negative connotation, as opposed to something more positive like “sharing investment advice”. The implication is usually that the “shill” is looking to make a quick profit, though it usually really just means that the person complaining about the “shilling” has a negative opinion of the project.