Seasonal indices provide a method to de-seasonalise data.
The seasonal index of a season/month/period/etc. compares the average value of a particular season to the average of all seasons in a cycle.
A seasonal index of 1 indicates the average value of the season is exactly equal to the average value of the entire cycle.
A seasonal index greater than 1 indicates the average value of the season is greater than that of the entire cycle (e.g. a seasonal index of 1.2 indicates the season’s average is 20% higher than the cycle’s average).