Osmosis is the movement of solvent molecules (in this case, water) across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration.
In the case of the gummy bear, it is made up of a polymer network (gelatin) that has small spaces between its molecules. When the gummy bear is placed in plain water, which has a lower concentration of solutes compared to the concentrated sugar solution inside the gummy bear, water molecules move through the gummy bear's membrane (which is permeable to water but not to larger sugar molecules) to equalize the concentration of solutes on both sides of the membrane.
As water molecules enter the gummy bear, the gelatin polymer swells as it absorbs water. This influx of water causes the gummy bear to expand and become larger and softer.
In summary, the gummy bear swells in plain water due to osmosis, where water molecules move from an area of lower solute concentration (plain water) to an area of higher solute concentration (inside the gummy bear), causing the bear to absorb water and expand.