Fantasy value doesn't have one concrete meaning. In season-long leagues, one might say that value is dictated by a player's total or weekly output versus their average draft position. In daily leagues, most players are measuring value by a player's output per $1,000 in salary -- in other words, the bang you get for your buck.
Finding cheap players in high-scoring games or with big matchup upside is pivotal to paying up for high-priced players such as Patrick Mahomes and Alvin Kamara. The combination of the two is what can drive tournament-winning lineups or ones that will help you cash (in 50/50s, etc.