When a substance is dissolved in a solution, it becomes a solute. Now when that solute spreads in the solution, that act is called diffusion. So diffused in this case means it spread out in the solution. Eg. Sugar cube in water. Sugar is a substance that dissolves in water( the solution) and it becomes a sugar solute. It then spreads in the water. That means it spreads from an area of more concentration to an area of lesser concentration. More concentration is the sugar cube, less is the water before being mixed evenly with the substance/sugar.
When a substance is able to pass through a membrane without restriction, it means the membrane is permeable. In contrast, a selectively permeable membrane only allows some solutes to pass through and not others.