A solvent is a substance that is able to dissolve other substances.
Water is called the "universal solvent" because it is capable of dissolving more substances than any other liquid.
This is important to every living thing on earth! It means that wherever water goes, either through the air, the ground, or through our bodies, it takes along valuable chemicals, minerals, and nutrients.
It is water's chemical composition and physical attributes that make it such an excellent solvent. Since water molecules are polar, they are attracted to many other different types of molecules. Water can become so heavily attracted to a different compound, like salt (NaCl), that it can disrupt the attractive forces that hold the sodium and chloride in the salt compound together and, thus, dissolve it.
This diagram shows the positive and negative parts of a water molecule. It also depicts how a charge, such as on an ion (Na or Cl, for example) can interact with a water molecule.