Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of lower water concentration (down a concentration gradient)
A partially permeable membrane is just one with very small holes in it. So small, that only tine molecules (like water) can pass through, and bigger molecules (e.g. sucrose) can't
The water molecules actually pass both ways through the membrane during osmosis. This happens because water molecules move about randomly all the time
Because there are more water molecules on one side than on the other, there's a steady net flow of water into the region with fewer water molecules, i.e. into the stronger sugar solution
Due to osmosis, the strong sugar solution gets more dilute. The water acts like it's trying to 'even up' the concentration either side of the membrane
Osmosis is a type of diffusion - passive movement of water particles from an area of higher water concentration to an area of lower water concentration