What does a low water potential mean?
Water potential is the "preference" of water to move from one place to another, and is made up of a lot of factors. For example, if water is at the top of a ramp, it has a high water potential due to gravity (it would tend to move downhill). If pure water is placed on one side of a permeable membrane, and a very salty solution is placed on the other, then the pure water has a high water potential due to osmosis (the pure water will tend to cross the membrane to equalize the salt content on either side of the membrane).
If there is a low water potential, then this means that there are few forces driving the water to move from one place to another, and the water will tend to remain as is.