Privacy means to be secluded from the sight, presence or intrusion of others. The adjective private is an opposite to public. Others is typically defined as those not involved. Typically it can mean directly involved but sometimes people consider family members involved even if indirectly such as private life.
Privacy for sexual activities
Some activities are typically done in privacy. A typical example is sexual intercourse in humans. Masturbation is usually done in privacy, too. Even a sexual orgy is private because all present are participating with one another, and there are none just watching.
Other things humans prefer privacy for include (naked) baths and showers and toilet use. Non-human animals, including apes, usually find no need for privacy for sexual activities, but some species prefer certain places and times of day for safety reasons. It is unsure how much of human modesty about nudity, sexuality and body functions is innate and how much is due to cultural conventions.
To ensure privacy, doors can be locked and windows can be closed.
Privacy in spanking and BDSM
In consensual spanking, private or public spankings may be given. The spankee may feel humiliation in private, but such feelings are heightened in public. In such an act, watchers observe as the buttocks are bared, displayed and spanked. Such a scene may be sexaully arousing to some, including the spankee.
"Reserved for one's exclusive use" is another meaning of private, e.g. a private home. In the BDSM context, a slave may be reserved for its Master's exclusive use. Thus, the slave is no longer available to a 3rd party. Essentially, a submissive agrees to be taken by a Master as private property. A Master may own other slaves, but a slave is bound in servitude.