The active side is that location. It's the three dimensional region found on that enzyme that is responsible for actually binding onto that substrate. So inside the active side so if this is our enzyme, this is our active side. This three dimensional crevice, the three dimensional crack in that enzyme is the active side. And this active side consists of the residues, those amino acids that are responsible for binding onto the substrate. And not only that, inside the active side, we also have these catalytic groups, these residues part of the enzyme that are responsible for actually catalyzing that particular reaction.