The phrase "accident of birth" refers to something that happens by chance or happenstance, rather than being a result of deliberate choice or action. It is often used in the context of discussing factors such as social class, race, gender, and other aspects of an individual's background that are determined by circumstances beyond their control. The phrase suggests that one's place in society, for example, is not necessarily a reflection of their abilities or choices but rather a result of where they were born and the opportunities (or lack thereof) available to them as a consequence.