Water-soluble preservative. This preservative is considered safe for use at concentrations up to 0.5%, although it is usually present at lower concentrations because it's typically part of a blend with other preservatives (such as parabens).
Diazolidinyl urea can be a formaldehyde-releasing preservative. Although that sounds scary, the amount of formaldehyde released is well below the recommended limits of exposure. Moreover, other ingredients (such as proteins) in a product cause the free formaldehyde to evaporate and become inactive before it could possibly harm skin.
See formaldehyde-releasing preservative
References for this information:
Steinberg, David C., Preservatives for Cosmetics, Third Edition, Allured Books, 2012, pages 67-68
Contact Dermatitis, August 2011, pages 81-91; and January 2006, pages 50-58
International Journal of Toxicology, March-April 1990, issue 2, pages 229-245