how to get priod of a given signal using autocorrelation in python

Question

i have written this function

def autocorr(x):
    result = np.correlate(x, x, mode='full')
    return result[int(result.size/2):]

corr=autocorr(PPG)


plt.plot(corr)

i have recived this plot.so what implies our periods, does it the peaks, the max or the min. can anyone explain this further and why does the period is found by using autocorrelation.