Hypertonic is more concentrated than isotonic, which is more concentrated than hypotonic.
This is all based on osmosis. The cell membrane is impermeable to the solutes in both fluids, blocking all passage of salts and other solutes both into and out of the cell. Ideally, the interstitial fluid will be isotonic with the intracellular fluid. That way water will not flow into or out of the cells.
Put simply, the cells are endangered if either hypotonic or hypertonic conditions exist. Hypertonic solutions cause water to leave the cells. Cells need water to operate normally. To do their activities, they need a cytoplasm medium, and a lot of these activities include a lot of different chemical reactions.