甚麼是 Hypertonic solution?

First of all, I assume you know that water potential is the highest for pure water (zero), and it becomes less for higher concentration of solute. For a body of an organism, there is a certain concentration of many types of soluble materials (such as salt). Hypertonic solution is a relative noun to the body fluid. If there is a solution which solute concentration is higher than the body fluid, then the former will be called a "hypertonic solution".

For your second question, I think you mean how a hypertonic solution can reduce the water potential of a body. If the hypertonic solution is mixed with body fluid, the former will diffuse the soluble materials to the latter, thus reducing the water potential. That is diffusion. If a hypertonic solution is separated from the body fluid by a selectively permeable membrane, as the soluble materials cannot pass through the materials, the hypertonic solution will absorb the water of the body fluid such that the water potential of the body fluid will be reduced until both liquids have the same water potential.

Hypertonic solution: A solution with a higher salt concentration than in normal cells of the body and the blood.

Hypertonic Solutions: contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm). When a cell is placed in a hypertonic solution, the water diffuses out of the cell, causing the cell to shrivel.
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm

When a plant cell is placed in a hypertonic solution, the water in the cells moves to an area higher in solute concentration, and the cell shrinks and so becomes flaccid [pron. flaxid]. (This means the cell has become plasmolysed - the cell membrane has completely left the cell wall due to lack of water pressure on it (the opposite of turgid)).
http://en.wikipedia.org/wiki/Osmosis

From the above definitions of hyperonic solution, you can see that the hypertonic solution is in higher solute concentration (which means the water molecules are lesser than normal and lesser water molecules implies lower water potential)

If a cell put in such kind of solution, it shrinks (which means water diffuses across cell membrane to the hyperonic solution and the water potential of the cell becomes lower).

A picture of osmosis can help you see this concept clearly which is in the following website: http://www.biologymad.com/cells/cellmembrane.htm