This is the 6472nd most frequent Korean word.
여
"여" means "female" or "woman" in certain contexts, but it can also be a part of a word or phrase without standalone meaning.
In this sentence, '여기' (here) uses '여' as part of a location particle, meaning 'this place.'
여기서 기다려 주세요.
Please wait here.
Here, '여동생' (younger sister) uses '여' to signify gender, specifically female, in the term for one's younger sibling.
어제 여동생을 만났어요.
Yesterday, I met my younger sister.
In this case, '여름' (summer) incorporates '여' as part of the term designating the summer season.