Wifi range extenders catch the wireless signal from your router then rebroadcast it, helping to strengthen the signal in another area of the house. Ideally you should place your WiFi extender halfway between your main router and your intended wireless devices, so it can catch a strong signal and extend its range. Most are wall plug range extenders that work instantly as soon as you plug them into a wall socket. Some require setup using an app on your phone or tablet.
Wifi extenders are considered an older technology, though still widely available and cost effective. When using extenders, you often have to manually connect to the closest extender which can be inconvenient if you have devices you that move around the home such as laptops, phones or tablets.
A modern solution exists and is coming soon to Broadway Broadband called Whole Home Wifi which you can find out more about here.