“On the rocks” means serving an alcoholic beverage in a cocktail glass with ice. Adding ice to a drink affects a drink primarily by keeping it cold and slightly diluting it over time.
A slang term for a drink with ice. A drink ordered “on the rocks” simply means a spirit, usually 2 oz, served over ice. Rarely would you drink a mixed cocktail without ice since most are either stirred or shaken with ice in order to be enjoyed at optimal temperature.