The Annual Rate of Return tool calculates the compounded growth over a defined period - also known as Compounded Annual Growth Rate (CAGR). In this example, the ASX200 index has grown 25.31% over the last 3 years, which is an annual rate of return 7.81% per year ((1+0.0781)^3)-1 = 25.31%.
This is a companion discussion topic for the original entry at https://www.optuma.com/kb/optuma/tools/price/annual-rate-of-return