It means you can go into the shop and connect to the internet on your phone or tablet, even computer and it doesn't cost you anything. Normally if you want to connect away from your own internet connection you have to pay for it.
It could be that you can use M&S WiFi signal in their shops to use your own devices i.e. laptop in the cafe or tablet, iPad or mobile phone.
WiFi in simple terms is a means of connecting devices without the need for wires or cables. For example I can connect my computermabob to my printer without cables and still print because the WiFi use my home router network settings.
Bluetooth devices work in a similar way without cables to talk to each other.
Your router is probably wifi enabled. Wouldnt you like to be able to connect from anywhere in your house, or have same facility in your pocket to connect everywhere except under water.
Think of saved space from a tin box on your desk to a credit sized gadget in your pocket ;)