If you mix two substances and the result is a homogeneous mixture, you are dealing with a solution.
In the case of table salt mixed with water, Na and Cl atoms, initially bonded together in the form of a crystal, are dissolved by molecules of water. Water is a solvent.
The reasons are electrostatic in nature. The cohesion of atoms and molecules derive from electrostatic links between particles that are charged or polar. Sodium chloride (NaCl) is in fact the joining of an Na+ ion and a Cl- ion, which mutually attract one another via electrostatic attraction.
Water molecules are electrically neutral, but their geometry causes them to be polarized, meaning that the positive and negative charges are positioned in such a way as to be opposite one another. This property makes the Na+ and Cl- ions break apart under the stronger attractions provided by the water molecules. Note that the orientation of the water molecules is not the same when it is attracting an Na+ ion as it is when attracting a Cl- ion.
This process continues until the salt is totally dissolved.