Hangul syllable ku

It contributes a sound; the full word supplies the meaning.

Korean uses 쿠 as one syllable inside longer words. In 쿠키 (“cookie”) and 쿠폰 (“coupon”), it is the opening syllable; the rest of each word makes the meaning specific.

Hangul was created at the Joseon royal court, a Korean dynasty, in 1443 and officially promulgated in 1446. It was designed to make writing Korean more accessible than relying on Chinese characters. Hangul letters combine into syllable blocks, which is why 쿠 appears as one block.

Why does 쿠 appear as ㅋㅜ?

A Korean input method has not joined the pieces.

On a Korean keyboard, ㅋ and ㅜ are entered separately; a Korean input method normally joins those keystrokes into 쿠. If you see ㅋㅜ, the text has kept the consonant and vowel letters apart. They are the input pieces, but the literal two-letter sequence is two characters, not the single syllable character 쿠.

For 쿠 vs 구, both contain the vowel ㅜ, but 쿠 starts with ㅋ, a consonant pronounced with a strong puff of air, and 구 with ㄱ, pronounced with less air. They are different syllables.