Briefly, some languages like English treat /g/ and /k/ as different sounds (phonemes), but other languages like Korean treat [g] and [k] as variations (allophones) of /k/ a single sound (phoneme). In general, a Korean consonant will be voiced when between vowels. Thus, [g] between vowels, and [k] otherwise. More information: https://en.wikipedia.org/wiki/Korean_phonology [excerpt] 3.^ /p, t, tɕ, k/ are voiced [b, d, dʑ, ɡ] between voiced sounds but voiceless elsewhere.

Korean consonants have three principal positional allophones: initial, medial (voiced), and final (checked). The initial form is found at the beginning of phonological words. The medial form is found in voiced environments, intervocalically and after a voiced consonant such as n or l. The final form is found in checked environments, such as at the end of a phonological word or before an obstruent consonant such as t or k. Nasal consonants (m, n, ng) do not have noticeable positional allophones, though ng cannot appear in initial position. ㄱ Initial allophone : /k/ ex) 가을 Medial allophone : /ɡ/ ex) 사과 Final allophone : /k̚/ ex ) 직진 ㄷ Initial allophone : /t/ - approximately t t ex) 다음 Medial allophone : /d/ - approximately d ex) 지도 Final allophone : /t̚/ ex) 닫기 ㅂ Initial allophone : /p/ ex) 바다 Medial allophone : /b/ ex) 가보 Final allophone : /p̚/ ex) 답가 ㄹ Initial allophone : /ɾ/ ex) 리본 Medial allophone : /ɾ/ ex) 나리 Final allophone : /l/ ex) 솔개 ㅈ Initial allophone : /tɕ/ ex) 자연 Medial allophone : /dʑ/ ex) 여자 Final allophone : /t̚/ ex) 잦다 Refer to https://www.youtube.com/watch?v=0ZhOeA0RD9o