The initiative

Typically, we understand an “advantage” as something tangible, at least for the coming 10-15 moves; e.g. more space, a weak enemy pawn, the bishop pair, the opponent's unsafe king, etc. In contrast, the initiative easily evaporates within a few moves if not sustained.

Thus, the side with the initiative typically tries to create new threats all the time, and doesn't shy away from material or positional concessions in order to keep the initiative going. All this might sound a little abstract, so let's look at some examples.