The term is the length of the mortgage.
The first option will reduce the capital amount (the amount you actually owe) and hence the term. In this option the money is "spent", so you can not withdraw it again.
The second will sit in an account earning interest and will be offset against your mortgage interest. This will not reduce the term, but will mean you pay less interest per month. The money is also available to be withdrawn later, which will obviously increase the amount of interest you pay month to month again.
The reason it will probably end up paying off your mortgage in full is that, assuming you keep overpaying, eventually the account will build up to such a level and the mortgage will reduce to a point that they converge. i.e. you will have overpaid so much that you can use the banked money to make a once off lump sum payment to clear the mortgage.