The index gives an indication of how favourable conditions are forecast to be for gardening.
The best is 10, 0 is the worst. The index is generated by considering forecast temperatures, wind speed, rainfall etc. For example, if it is too cold or too warm this will score the index down, likewise if it raining.
You may also want to check out the standard forecasts for this location.
Daily weather by email
14 day outlook
Monthly outlook
Seasonal outlook
Christmas weather
Charts and data