Under lattice formation energy, we talked about ions forming an ionic solid, but in lattice dissociation energy we're doing the opposite. Here it's the change in energy of 1 mole of solid crystal as it's scattered into gaseous ions.
So here we would have our sodium chloride solid and what would happen is we would split it up into its ions here, so gaseous sodium ion and gaseous chloride ion. This reaction is endothermic. Here we absorb energy in order to break the bond in order to split it up into its ions, and because we're absorbing energy.
That's why ΔH now is positive in terms of sine. Now we're going to say here, the more positive your lattice energy value, then the more endothermic the reaction will be.