I found this quote in some Geography IBL documents and I think it nicely and easily sums up IBL for the new student Tag Line you are after:
IBL refers to a range of approaches to learning that are based around processes of self-directed inquiry. Students conduct inquiries which are designed to enable them to actively explore questions and problems within their discipline.
Its major points are that IBL is proactive and self directed and involves the process of actively exploring questions and problems
Source: natalie whelan