What does a low water potential mean?

A low means that water has a low force driving it to move from one area to another

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 (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.

There's a good review on Wikipedia, here