Copper Sulfate can dissolve in water because water is a polar solvent. A polar solvent is one where the molecules that make up the solvent have a charge that is unequally distributed in the molecule. In the case of water the oxygen atoms have a partial negative charge and the hydrogen atoms have a partial positive charge.
Copper sulfate is ionic in nature. When it is dropped in water the copper sulfate dissociates into positively charged copper ions and negatively charges sulfate ions.
The polarity of water leads to the positive copper ions being attracted to the oxygen atoms of water with a partial negative charge and the sulfate ions being attracted to the hydrogen atoms of water that have a partial positive charge.
Water being a polar solvent can easily dissolve ionic salts like copper sulfate.