Invert sugar is produced when sucrose is split into glucose and fructose. It is used in confectionery, baking and frozen desserts to help control crystallization and retain moisture. Commercial invert syrup has a specified concentration.
It is not simply any sugar dissolved in water: ordinary simple syrup may still consist largely of sucrose. Syrups differ in water content and composition, so replacing one with another can change sweetness, texture and freezing behavior.
Reference: Further reading.