Personal Space refers to the physical space around our bodies that we like to keep free; we tend to only let close family and friends, pets, and intimate partners enter our personal space. When anyone else enters, we feel uncomfortable and possibly threatened, so we tend to move or back away to increase the distance.
The exact amount of personal space we need depends on personal preference and cultural background. According to Edward T. Hall, the average radius of intimate space is 1.5 feet. In densely populated areas, people may be used to a smaller amount of space. If you've traveled internationally, you may have noticed that different countries have different norms for personal space.