What is the concentration gradient in osmosis?

high to low concentration of solutes.  Water goes from high to low to balance everything out

Water moves from high water to low water which is another way of saying from low solute to high solute.

osmosis is simply diffusion. I remember being confused by it when I was learning about it, but its really quite simple.

lets say you have two chambers separated by a semi-permeable membrane. All that semi-permeable means is that some stuff will go through it and some stuff won't. Generally water will pass through while whatever is dissolved in the water (the solute) won't.

When a substance diffuses it gradually spreads out from an area of high concentration because of random movements of its molecules. After a while it will be evenly spread throughout the solution. You can liken osmosis to diffusion of water.

In our two chambers one side is pure water, and the other side is water with a solute dissolved in it. The solute cannot leave the chamber because the membrane is impermeable to it while the water can go wherever it wants.
If you look at the "concentration of the water" (though that is a strange way of saying it) the water will be at 100% concentration in the pure chamber, and somewhat less in the solute chamber. This is because with other stuff dissolved in it the water can never be at 100% concentration. Some of the liquid will always be the solute.
What we have is a gradient or water concentration that the water will tend to diffuse down and try and make the water concentrations in the two chambers equal. The water will diffuse across the membrane to the solute chamber just like dye dropped into water diffusing from high to low concentration.

If the "solute" chamber is a cell (generally people are talking about cells when they talk about osmosis), then the "pure" chamber is the outside of the cell. Of course there is stuff dissolved on both sides, but a cell is full of lots of different solutes. In fact a cell is just a bag to keep those solutes in really. There is therefore usually a tendancy for water to move into the cell from the surrounding fluid and cause the cell to swell. This can be seen if you put a cut section of potato into tap water for a few days - the section will swell slightly and you can measure this. Thats the standard teaching experiment.

It seems you have fallen into the trap of not knowing what concentration gradient they are talking about. Some sites will be talking about the solute concentration, some about the "concentration of water". As the water is whats moving focus on that. If the solute conc. is high then the water conc is low and water will try to move into that area across a membrane if the water conc. is higher on the other side.

The reason all this is important is that if the concentration of solute is too high outside the cell water will tend to flow out of the cells and they will deflate (cell crenation) and cause important life processes inside the cell to be disrupted. If the solute concentration is too low outside the cell then water will rush into the cells and lead to the cells bursting. This is why you have kidneys and a sense of thirst - they work together to keep the concentrations just right for your cells to survive happily.

OK, so maybe it isnt that simple, but once you get it it will just click in there I promise.