Typically those delivering no-limit hold'em advice highlight the benefits of playing an aggressive game and emphasize the problems that come from playing too passively. It's true that in many situations — both in tournaments and cash games — being aggressive is going to be a more advantageous style, but that doesn't mean you should never play passively in a given hand or situation.
Being "aggressive" typically refers to betting or raising when given the opportunity rather than taking the "passive" route of checking or calling. Beginning players sometimes mistake the advice to "be aggressive" to mean they should always be betting or raising, and that checking or calling automatically marks them as weak, passive players.