What is solute potential?
Solute potential (also known as Osmotic potential) is shown with this symbol: Ψ
But getting to your question, solute potential is a component of water potential. It happens because solute molecules are present. It is always negative since solutes lower the water potential of the system.
So if you fully want to understand solute potential, here is a quick explanation of water potential. Basically, water potential is the energy of water unit volume relative to pure water that you can reference. This also affects water's tendency to move from one area to another due to osmosis, gravity, mechanical pressure, or other cool stuff. All though it's mainly done IN plants, it can happen other places as well.