Behavioral design means influencing what people do (behavior) by modifying (designing) the environment in which the behavior takes place.
The changes in context (aka. behavioral interventions) can have different levels of firmness - the extent to which they restrict the users' freedom exhibiting behaviors that are different from the one desired by the designers.
At the first level of firmness, the changes in the environment are subtle and aim at guiding behavior.
For example, the do not disturb door-handle tag in hotels is a relatively subtle cue of what the desired behavior is – do not come into the room (to clean it).