Simply put, diffusion is the movement of molecules down a concentration gradient.
The purpose of diffusion basically is just to even out the concentration of molecules across the space.
When you first drop the food coloring into the fishbowl, it'll all be clumped up in that space right where you dropped it; but slowly, those molecules are going to start to spread out. They're going to move to an area of high concentration, right where they were dropped into an area of low concentration.
The rest of this for this fishbowl doesn't have any green food coloring molecules so those molecules are going to spread out from that area of high concentration to that area of low concentration.
When diffusion is done, those molecules will be equally spread or equally distributed throughout the water in that fishbowl. So you would notice, when you first dropped the food coloring in, you would see just this area of green, but after diffusion occurs, all of the water would then be green, and you would know that those molecules had spread out evenly, or diffused evenly, from an area of high to low concentration.
Below is another image example in a cell. Outside of the cell, you might have a certain solute that you have in high concentration. You have a lot of it outside of the cell, but you don't have very much of it inside of the cell.
Naturally, those molecules are going to diffuse from an area, again, of high concentration to low concentration. You're going to see these molecules outside of the cell diffuse across the plasma membrane into the cell.
Again, when diffusion has happened, you would have an equal number of molecules outside as you would inside. These will continue to diffuse into the cell until it's equal on either side of the cell.
Source: this work is adapted from sophia author Amanda Soderlind.
The difference in concentration of a substance between two areas
Movement of molecules from an area of high concentration to an area of low concentration
The movement of molecules across a membrane without the use of energy