While "engagement" might seem a bit amorphous, Brad focuses his comments on navigating between the poles of neo-Anabaptist cultural withdrawal and "Christian Right" political activism. In other words, in an increasingly non- or anti-Christian cultural moment, Christians must avoid the twin (but opposite) perils of retreating into the Church to flee from cultural engagement and of disconnecting cultural (including political) activity from the Church. (And, if I had a third hand, of limiting the Church to the clergy or even to the assembly of the saints in worship.)
If my brief description leaves you mystified, simply watch the interview. It's only eight minutes.
One more thing. This interview has the best one-liner of the series: "We're dealing with human beings not just disembodied bundles of ideology." In other words, when working in a society that rejects the transcendent norms recognized by Christians, we must remember that the norms are true and evident at some basic level even to those who ostensibly deny them.