Percent chance of rain?

A forecast that says, for instance, "there is a 10% chance of rain tomorrow," means that on 10 out of 100 days with weather conditions very similar to the conditions expected tomorrow, at least a trace of train has occurred.

Some will disagree with this answer for various reasons. For the results of research that evaluated how understanding of percentage chance of precipitation varies around the world, see "A 30% Chance of Rain Tomorrow: How Does the Public Understand Probabilistic Weather Forecasts?," available as a PDF file via the URL http://www.mpib-berlin.mpg.de/en/mitarbeiter/gigerenzer/pdfs/RainFinal.pdf .