A "kouhai" is your junior at work, school, or a group. Usually they belong to the same organization as you but they entered the organization after you. A "kouhai" could also be someone who has entered the same "industry" as you but at a later date. In this case "industry" has a very broad meaning, e.g. the entertainment industry, a professional sports league.
The Sempai/Kouhai relationship can be quite confusing and complicated. A "sempai" is the person with more experience at a job or longer tenure in an organization/group. The "sempai" is usually older and wiser (at least they're supposed to be wiser. They've been around the block a few times, so they are supposed to know the ropes. It is the sempai's responsibility to look out for their kouhais. To mentor them and help them get used to their new surroundings. Conversely, a kouhai is supposed respect the sempai and learn all they can from them. They are supposed to defer to the sempai and their experience is certain situations. Eventually, kouhais become sempais so they will get the chance to be the big shot. Someday, they will get their chance to sit on top of the totem pole.
The kanji characters for kouhai are 後輩. 後 is read as "kou". It means "behind", "after", "later", etc. The character for "hai" is 輩. It means "companion" or "line" as in things (people) in a line. So, I guess "kouhai" could literally be translated as "later companion" or "person in line behind me".