When you remortgage, you are basically making a switch from one lender to another, using the proceeds from the new mortgage you got to pay off the old mortgage from the previous lender. For both, you are using the same house/land which up put up as security for the mortgage. This process is not so common in India, though.
A person may use remortgage to take advantage of the initial low offer given by a lender, switching over once the duration of that offer is over to then take advantage of the initial offer of another lender. This thus reduces the overall costs.