Supposedly, a collection of sensors.
Greeblies like that are the Sternbach way of showing exposed instrumentation. Andrew Probert's Enterprise-D has such things in the trench around the saucer rim, but Sternbach expanded from there and applied sensor farms like that on various exposed positions on the TNG shuttlecraft, DS9 runabouts, and eventually also Voyager.
The sensor farm is never seen in "action" (whatever that might be) or referred to in dialogue, but that's how it's always labeled in backstage material, scale model kit instructions etc. There's also a smaller rectangular counterpart aft of the bridge, and several lateral slits on the primary and secondary hull rims.
Timo Saloniemi