If the sun is shining on the ice, some of that solar energy will be absorbed. Ice is clear at visible wavelengths, the energy to which our eyes are sensitive. But the sun emits radiation at other wavelengths which water will absorb, and thus increases its energy gain.
If these gains exceed the energy losses, the ice will warm. If it reaches the melting point, the ice will start to melt. The sun can also add energy and warm the sidewalk, increasing its temperature to above freezing. This will then warm the ice via conduction and lead to its melting.
Hope this helps lol