Kouhai (こうはい) means your junior: someone who joined your school, club or workplace after you.
If you know せんぱい, you already know half of a pair. こうはい is the other half: the person who came after you.
What it actually means
Your こうはい is anyone who entered your school, club or company AFTER you. As with せんぱい, age doesn't matter — entry order does. Join a company at 40 and the 23-year-old who started last year is your せんぱい, and you are their こうはい.
The one rule: you don't call people こうはい
Here the pair is not symmetrical. You can call someone せんぱい to their face, like a title. You never call someone こうはい to their face. A senpai simply uses the junior's name — very often the bare name with nothing after it, or with くん or ちゃん.
ふじせんぱい、 おはようございます！
Good morning, Fuji-senpai!
おはよう、ゆき。
Morning, Yuki.
Notice the two styles in that little exchange. The こうはい uses polite speech (おはようございます) and the title. The せんぱい can answer casually (おはよう) and uses the bare name, ゆき, with no さん. Dropping さん like that is normal from a senpai to a kouhai; the other way round it would be rude. That difference in politeness is how you hear who is who in an anime club room, even before anyone says せんぱい.
What each side gives
The こうはい shows respect, listens and often does the small jobs. In return the せんぱい is expected to teach, look after the newcomer and, quite often, pay for the meal. It is a relationship of care in both directions, which is why stories love it.
Can you use it?
Yes — for describing. People usually say where the two of you were together: かれは わたしの だいがくの こうはいです (“he was my junior at university”) is ordinary, friendly Japanese. Just remember the rule above: talk ABOUT your こうはい, and talk TO them by name.