Imagine your local municipal water authority down in a trench, digging to repair a waterline that's buried ten feet below the street. Now, imagine that for some reason the federal mandated trench protection fails and the trench collapses, trapping men inside. How do you get them out without causing further collapse and endangering rescuers. That's what trench rescue is...
As far as what certifications are required... This is a basic class in trench rescue, so in theory if you're taking it you don't have any prior certification in it. It does list a pre-requisite, which, according to their course catalog that MMiz posted, is Low-Angle Rope Rescue, so if you haven't taken that class you're S.O.L.