In Japanese, kouhai 後輩 means "junior," in the sense they've been in a workplace, organization, school, or school club, for less time than you have. It can also refer to someone who has been doing an activity, like a sport, for less time than you.
The word kouhai is also romanized kōhai. It's the antonym of senpai 先輩, "senior," as one might guess from the meaning of the kanji of the words: saki 先 means "early," while ato 後 means "later." A senior is the one who joined earlier, before, the junior who joined later.
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