The phrase "to be in" is an idiomatic expression that means to experience or undergo a particular emotion, state of mind, or physical sensation. It can also mean to participate in or engage with something. For example, if someone says they are "in love," it means they are experiencing the emotions and feelings associated with being in a romantic relationship. Similarly, if someone is "in pain" it means that they are physically feeling discomfort or suffering. The phrase can also be used to describe a state of mind or physical presence, such as saying one is "in their room," meaning they are currently present in their bedroom.