解开 is a verb-resultative compound (解 'to untie, loosen' + 开 'open'). It means to undo something that was tied, fastened, or closed, and extends metaphorically to solving puzzles or dispelling doubts. The specific English equivalent depends on the object: literal knots and fastenings (untie, undo), abstract problems (solve, figure out), or emotional/mental states (dispel, relieve).