Basically, the shields they used would only block things with at least a certain threshold of energy or speed.

From the Wookiepedia entry on shields:

The Gungan shields used during the Battle of Naboo blocked lasers, but droids could pass through because of their relatively slow movement in comparison to the laser bolts.

Basically, the shields they used would only block things with at least a certain threshold of energy or speed.

From the Wookieepedia entry on shields:

The Gungan shields used during the Battle of Naboo blocked lasers, but droids could pass through because of their relatively slow movement in comparison to the laser bolts.