What is the importance of osmosis in maintaining the structure of a plant?

Osmosis is the passive movement of water molecules down their concentration gradient across a partially permeable membrane. In plant cells, water moves via osmosis into the vacuole of the cell. The osmotic pressure (i.e. the pressure created by having lots of water molecules in the vacuole) pushes out, and makes the cell turgid. Maintaining the turgidity of cells helps to keep the plant upright (imagine the difference between a bouncy castle that's full of air versus one that's started to deflate). This is necessary in plants, because unlike in many animals, they don't have bones to keep them upright.