A 15% chance of rain indicates there is a 15% probability that measurable precipitation (at least 0.01 inches) will fall at any specific point within the forecast area during the specified time period.

This percentage is derived from two factors:

Forecaster Confidence: The meteorologist's certainty that precipitation will form or move into the area.
Areal Coverage: The percentage of the forecast area expected to receive rain.
For example, a 15% chance could mean the forecaster is 100% certain it will rain, but only over 15% of the area, or 15% certain it will rain over 100% of the area. It does not mean it will rain for 15% of the day or that 15% of the population will get wet.