An isotonic solution is a solution in which there is no difference in the concentration gradient of materials present inside and outside of the cell. Isotonic, hypertonic, and hypotonic refers to what happens to materials during passive transport. It could be the movement of water (osmosis), or other "stuff" (diffusion).
If a cell is placed in an isotonic solution, that means the amount of stuff inside the cell and outside the cell is equal. For example, let's say that you have a cell that is 80% water and 20% salt. If you placed that cell in a beaker that contained a solution of 80% water and 20% salt, then the concentration levels are equal. No diffusion or osmosis will occur. That's because passive transport mechanisms seek to reach equilibrium. The above example is already at equilibrium. There will be some movement of water and salt across the cell membrane, but there will be no net concentration change. The cell will remain the same in an isotonic solution.
In a hypotonic solution, material will flow into the cell, which will cause the cell to expand and swell. A hypertonic solution has material flowing out of the cell, and the cell will shrivel.