Salience basically means "stand out-iness". E.g. a salient issue is an issue that stands out as important. In addiction, it refers to how the thing you're addicted to is constantly at the top of your mind.

This paper describes it well:

> This refers to when the particular activity becomes the most important activity in the person’s life and dominates their thinking (preoccupations and cognitive distortions), feelings (cravings) and behaviour (deterioration of socialized behaviour). For instance, even if the person is not actually engaged in the behaviour they will be thinking about the next time they will be.