An isotonic cellular environment occurs when an equal solute concentration exists inside and outside the cell. Molecules flow in and out at an equal rate by osmosis, causing the cell size to stay the same. It will not lose or gain any solutes. Plant cells in an isotonic environment are flaccid (they lack firmness) and they will wither. The equilibrium of water movement is unable to provide plant cells with internal pressure for structural support.
An isotonic solution has an equal amount of dissolved solute in it compared to the things around it. Typically in humans and most other mammals, the isotonic solution is 0.9 weight percent (0.009 g/ml) salt in aqueous solution, this is also known as saline, which is generally administered via an intra-venous drip. It is a medium between hypotonic and hypertonic solutions.
Osmosis is the diffusion of a solvent through a selectively-permeable membrane from a region of low solute concentration to a region of high solute concentration, or in other words, from a high water concentration to a low water concentration. The higher the solute concentration the lower the water concentration will be. The selectively-permeable membrane is permeable to the solvent, but not to the solute, resulting in a chemical potential difference across the membrane which drives the diffusion. That is, the solvent flows from the side of the membrane where the solution is weakest to the side where it is strongest, until the solution on both sides of the membrane is the same strength (that is, until the chemical potential is equal on both sides).