Easy, check it out:
Breaking bonds requires energy. Some solids have bond enthalpies (energies) that are too big for water at room temperature to break, hence, if you supply heat to the water, it will have more energy to break the bonds of the solid, making it easier to dissolve.
There is another, more correct way to think about it, however, this requires you to know about Gibbs Free energy. If you have this knowledge about thermodynamics I can walk you through that (just comment below). But otherwise just go with the first paragraph.