Dimethyl isosorbide is a clear, slighlty oily-feeling liquid that functions as a solvent and may be plant-based or synthetic. It is also used to decrease the thickness of products, such as keeping a liquid in its fluid form rather than wllowing it to rever to a lotion (or, in cosmetics chemistry parlance, a thicker emulsion).
Another function of this ingredient is to allow controlled, sustained delivery of key ingredients such as antioxidants, so they can, in theory, provide stronger benefits.
References for this information:
International Journal of Pharmaceutics, October 2012, issue 1, pages 38-49
ChemSusChem, May 2010, issue 5, pages 566-570
Skin Pharmacology and Applied Physiology, issue 6, October 2008, pages 326-334