떨어지다 is falling, dropping or coming away, and after -아/어 it forms compounds: 나가 떨어지다 'drop out, be knocked out of it', 부딪혀 떨어지다 'crash and fall'. The one worth learning is 맞아떨어지다 — things matching up exactly: 우리 앨범 커버와 사진의 비주얼이랑도 맞아 떨어져요 'it lines up with the album cover's look too', 이런 패턴들이 맞아떨어지겠다 'these patterns are going to line up'.