Travaux Préparatoires = "[French 'preparatory work'] Materials used in preparing the ultimate form of an agreement or statute, and especially of international treaty; the draft or legislative history of a treaty." 
 Black's Law Dictionary 1638 (9th ed. 2009).
For further understanding of treaty interpretation and the use of travaux préparatoires, see:
Jonathan Pratter, À la Recherche des Travaux Préparatoires: An Approach to Researching the Drafting History of International Agreements, GlobaLex (Nov./Dec. 2012).
Martin Ris, Treaty Interpretation and ICJ Recourse to Travaux Préparatoires: Towards a Proposed Amendment of Articles 31 and 32 of the Vienna Convention on the Law of Treaties, 14 B.C. Int'l & Comp. L. Rev. 111 (1991).
Travaux Préparatoires, often referred to as travaux, are an important source when interpreting treaties because they can help elucidate the drafters' intent. Consulting travaux is an essential research step when treaty interpretation is at issue.