Remember, when a cell exists in a solution, the concentration of substance dissolved in the solution has a big effect on the cell. If there is more stuff dissolved in the solution than what is dissolved in the cell, then the solution sucks water out of the cell, killing it. If the solution has less stuff dissolved in it than what is inside the cell, then the cell starts sucking water in, and the cell eventually explodes.

In an isotonic solution, the amount of stuff dissolved in the solution is the same as the amount of stuff dissolved in the cell. This is the ideal situation for the cell, as water is neither sucked into nor out of the cell. The cell simply resides happily in the solution.