Now click on the Autocorrelation tab
The autocorrelation period estimate is the time of the lag at the peak near 24h, marked by a red asterisk in the autocorrelation graph. The correlation value at the peak is called the rhythmicity index (RI). A high RI near 1 indicates that each cycle is very similar to the others, so the time series is strongly rhythmic. Lower values indicate more variability from cycle to cycle, due to noise or fluctuations in period or amplitude.
Read the information in the ‘How does autocorrelation work?’ link to get some background.