Intervention is the term we use to describe any effort made to intervene in people's lives purposefully. That might be something very simple like a leaflet or a classroom curriculum, or it may be something much more complex, like training for kitchen staff in a school accompanied by changes in labelling in the canteen for the children.
So you can have very simple interventions or you can have very complex interventions. But it's trying to put a boundary around any program or any initiative that is being evaluated.