It simply means that . . .
"Gift Cards and E-gift Cards/Voucher codes:
may be used to purchase goods of a higher price than the amount on the card, on payment of the difference.
cannot be exchanged for cash or used to pay for M&S Bank services, products or outstanding card balances, made to measure shirts, personalised cards or M&S Energy or to buy another gift card or e-gift card.
can be used in all M&S stores in the UK and the Channel Islands, including Outlet, railway, motorway and airport stores but not in BP Connect stores
are valid for 24 months from the last transaction (transactions include balance enquiries). Any remaining balance will be cancelled on expiry of the validity period.
cannot be returned or refunded, except in accordance with your legal rights, for example in the event they are not of satisfactory quality.
should be treated as cash, M&S will not accept liability if lost, stolen or damaged."
http://help.marksandspencer.com/support/company-website/terms-and-conditions
This might help make your £100 go even further!
http://forums.moneysavingexpert.com/showthread.php?t=4341245