손 - hand 틈새 - crack/gaps, +로 - by means of 비치는 - breaking through (in terms of light), adjective form of 비치다 - to shine 내 맘(마음) - my heart 들키다 - to be discovered + 까 두렵다 - I am afraid that it will "I am afraid my heart will be seen through the gaps in my hands" is referring to the idea that the speaker is trying to hide their emotions but they're afraid that someone will see how they truly feel. (At least that is my interpretation of that sentence.)

lol. it's IU's song lyrics! am i right? kkkkk