Korean has a three-way distinction for consonants that most languages don't have: plain, aspirated, and tense. You've already learned plain (ㄱ, ㄷ, ㅂ, ㅅ, ㅈ) and aspirated (ㅋ, ㅌ, ㅍ, ㅊ). Now it's time for the tense consonants! These are written as doubled versions of the plain consonants. The secret to pronouncing them: tense your throat, and release the sound without any puff of air.