To understand the phenomenon, it must first be understood that the envelope of the soap bubble consists mainly of water… A hundred superimposed soap bubble membranes would be more or less the thickness of a sheet of paper. Then, on either side of this layer, there are surfactant molecules, similar to those used in dishwashing liquid. These molecules have two ends: one hydrophilic (which mixes well with water) and the other hydrophobic (rejected by water). In the film covering the soap bubble, the hydrophilic end is oriented towards the water molecules and the hydrophobic end towards the air.
When a drop of water crosses the film of the soap bubble, it dilutes the water/surfactant mixture somewhat to its point of entry. Since the water molecules like to stick to each other, the drop does not destroy the thin layer of water and simply passes through it.