An isotonic solution is when two solutions, separated by a semipermeable membrane, have equal concentrations of solutes and water. Imagine you're at a party and there are an equal number of guests in the living room and in the kitchen. It doesn't make much of a difference where you stand because you are equally as comfortable in either room. You have just as much space, you can move just as easily, and you have equal access to food. You don't spend a lot of energy trying to get out of one room or into another. This party is like an isotonic solution; everything is equal from room to room.
