Hi navin,
About "Biased Estimate":
you estimate the autocorrelation function with an average of products.
For the lag "m", there are N-|m| products involved.
If you divide the sum of products by N-|m|, the estimate is unbiased (its expect value equals the true value you are estimating) but its variance increases with |m|.
If you divide the sum of products by N, the estimate is biased (its expect value is not equal to the true value you are estimating) but its variance is smaller.
The subject is treated, for example, in the book "Digital Signal processing", 3rd Edition, by Proakis and Manolakis, but in other books too. Look for Estimation of Autocorrelation.
Regards
Z