sugers are in equilibrium between their open chain and cyclic forms in water. as a result if you can imagine a single, lets say glucose molecule, it would isomerizes between its two forms (cyclic and open cair). when the cyclic isomer is formed the hydroxy group on C5 can attack the carbonyl carbon from top or bottom. this can cause formation of an alpha anomer or beta anomer. this whole continuous process is mutarotation.
simply put its when a reducing suger ( a hemi-acetal ) opens up and closes back to form its corresponding anomer.