When you take a home loan from a bank you will be paying it back in the form of an Equated Monthly Installment (EMI). This EMI constitutes both the principal as well as the interest component. Now suppose you have got a windfall in the form of annual bonus or gift or anything of that sort and you would want to lower the burden of EMI's. In this situation you will pre-pay your loan i.e. deposit these surplus funds in your home loan account with the bank. This will reduce your principal component and eventually reduce your EMI's. Obviously, this prepayment is not good news for the banks and they try and dissuade you by putting extra charges on pre-payment.
Hope this helps.
Kris