"Run into someone" is an informal phrase that means to unexpectedly meet or encounter a person. This can happen in various places, such as on the street, at a store, or at an event. The meeting is usually casual and unplanned, often leading to a brief conversation. For example, you might run into a friend from school while shopping at the mall. This phrase can also apply to acquaintances or even strangers, making it a common way to describe chance encounters in everyday life.