Breath occurs when the vocal folds in the trachea are parted in order to allow air to enter and fill the lungs, or to exit. Usually the vocal cords are parted enough that no discrete tones are produced during breath.
For singers, breaths in are generally placed between phrases in a performance. For instrumentalists, breath marks are often placed between or within phrases as a way to articulate the musical line or to emulate vocalityor, indeed, in a place where the player of a wind or brass instrument actually needs to take a breath. The notation for a breath mark in Western musical notation is an apostrophe, usually placed at the top of the staff at the point where the “breath” is to be taken. This mark is also called a “luftpause.”