To solve something is to find a solution, like figuring out the answer to a complex riddle.
The verb solve is often used in mathematics, and it means to answer a math problem. You can solve other, non-mathematical problems too — like the question of what to wear when you've run out of clean clothes (your sibling's clothes!). The word solve originally came from the Latin solvere, which meant "to loosen or untie." If you think of any kind of complex problem as a knot, then the original definition of solve still makes sense!