Think of all the different screen sizes an devices you can browse the web on, have you ever browsed a website on your phone and it was horribly ‘broken’ and unusable? Something called responsive web design is meant to make that latter part never happen. A responsive web page is one that resizes and shapes itself based on the screen dimensions that you have when browsing, from the smallest smart phone to a 32 inch 4k monitor.
Responsive web sites make UX a pleasant and can make the website extremely valuable if an app isnt available. Say its 5pm friday you’re getting on the train from work and want to order a pizza, all you have is your smartphone. The local pizza shop does not have an app but it does have online ordering on their website. You browse to the site on your smartphone and either: A. The website will be responsive and you can easily order a pizza on your smart phone browser or B. the website is not responsive and an extremely frustrating experience to navigate around thus potentially making you not order.
Now days thankfully most websites are fully responsive with a lot of credit being given to bootstrap and their column system which really makes building responsive web sites so much easier and hassle free.