Thanks for the question. It actually says to add a die if you "exploit an advantage." It's not defined because it's not a game mechanics term here (unlike, say, in 5e). It's up to the GM and players to use the fiction established as they play to determine whether how a character is doing something should qualify as exploiting an advantage. Anything that has been set up in the game and invoked in the player's description of their actions that you feel should give them a significant boost in achieving their goal can qualify as an advantage.