An iso osmotic solution, which is more commonly called an isotonic solution, is a solution which has the same concentration of water as whatever it is being compared to.
Osmosis, which is the movement of water across a membrane, is a physical process driven by the concentration of solutes in the water on either side of the membrane. In the case of cells, we can predict whether water will move into the cell, leave the cell, or remain constant, as long as we know the concentration of water both within the cell and around it.
A hypertonic solution is one that has greater solute concentration (and therefore less water) than the cell; submerging a cell in a hypertonic solution will cause water to leave the cell, because the water will move toward the region where the water concentration is lower. A hypotonic solution is the opposite - it has more water than the cell, and a cell in a hypotonic solution will gain water and swell up.
An isotonic solution, because it has the same relative concentrations of water and solute as the cell, will be in equilibrium with it. A cell in an isotonic solution will neither gain nor lose water.