BDSM, or Bondage and Discipline Dominance and Submission Sadism and Machoism, has slowly developed into a catch-all term for a wide range of activities, relationship styles and more.
BDSM photography, or bondage photography, often explores the themes of BDSM - such as "sub and dom" relationships - in a modelled or simulated manner. As a result, engaging in this level of modelling requires a high level of trust between all of the creatives on the shoot to ensure that everything is discussed appropriately beforehand and runs smoothly on the day.
It's also not a level to be entered into lightly and should be done with significant consideration. This level will often require a model being tied up or otherwise restricted, and insertions.