The phrase "to go in" means to enter or to be admitted into a place, situation or organization. It can also mean to start working on something or to begin an activity. For example, if you say that someone is going in for surgery, it means they are entering the operating room and undergoing an operation. Similarly, when you say "to go in" for a job interview, it means you are entering the company's premises to meet with the interviewer. The phrase can also be used figuratively to describe someone starting a new project or taking on a new responsibility.