The lattice energy of a crystalline solid is usually defined as the energy of formation of the crystal from infinitely-separated ions, molecules, or atoms, and as such is invariably negative.
When #NaCl# dissolves in water, the crystals disappear on the macroscopic scale. On the atomic scale, the #Na^+# and #Cl^-# ions in the crystal are released into solution. The lattice energy of a salt therefore gives a rough indication of the solubility of the salt in water because it reflects the energy needed to separate the positive and negative ions in a salt.
Opposite of this picture is this example: The lattice energy of #NaCl# is the energy given off when #Na^+# and #Cl^-# ions in the gas phase come together to form the lattice of alternating #Na^+# and #Cl^-# ions in the #NaCl# crystal.
