Diffusion is the movement of particles down a concentration gradient. Osmosis is the movement of water particles. If there is no gradient then the particles will not move and therefore diffusion and osmosis will not take place.Think about a classroom full of students trying to pass through a door-it's easier for the people inside the crowded classroom to exit the room than for people standing in the almost empty halfway to get back into the room. Gradients are basically differences in concentration of a substance across a border. The concentration of people inside the classroom is higher than that of those outside the room (the border being the wall). The fact that there is a difference in "concentrations of people" in the 2 spots is what constitutes a concentration gradient. Osmosis and diffusion can happen without an input of energy (Such as from ATP), so the only way molecules can do this is by traveling down the concentration gradient. That is, they travel from high concentrations to low concentrations.
