If you overpay your mortgage, the balance will reduce.
If you overpay your mortgage, you will pay it off earlier than you would otherwise have paid it off.
"the overpayment ... will not reduce the term" probably means the following:
You owe €100,000 and you have 10 years left on your mortgage, so the repayment is €1,060.
You decide to pay €2,000 a month instead.
At the end of year 1, you will owe about €80,000 and so with 9 years left to go, the repayment due will reduce to €921.
So they left the term as it was, and reduced the repayment due.
If you continue to overpay your mortgage, it will be paid off early.
If, after a year, you are under financial pressure, your repayment will drop to €921.