In the past couple of years there's been a sea change in the devices used to access the internet. Smart phones and tablets have proliferated and continue to do so, to the extent that if you have or want to have a website, you should consider whether your visitors will likely use their phones or tablets to view your site. Why is this important? Simply because of the size of the screen, techincally known as the viewport. In the "old days" (just a year or two ago) we used to build sites with regard for viewport size, but the focus was on desktop computers, which kept using larger monitors and so the basic strategy was to build a site based on the a standard desktop size, then center the content on the page so that no matter how large the viewport, the content would look like it was appropriately sized. But then the viewports got small and sites that were built for desktop monitors were too big for small screens like phones and tablets. Viewing sites with the smaller devices required one to scroll left and right, up and down, all of which became pretty annoying to the viewer, as they weren't having a very good viewing experience. Neccessity being the "mother of invention", Responsive Web Design was suddenly very important, as the trend toward accessing the internet with smaller devices was in full swing, and likely to continue for the forseeable future.
Responsive Web Design can be accomplished in a number of ways that I won't go into here, but what you should know is that it enables a website to be viewed on any size of device without having to scroll left and right. Yes you will still have to scroll down, just as you will when viewing sites on a desktoop computer, but since we're used to this, we don't find that annoying. So the result is a better experience for the viewer, which can be important if your viewers are trying to find your product to buy or learn about your services quickly, hopefully not while driving their car.
Rather than go on with explaining details that you may have no interest in, let me just show you an example of how this works.
One of my favorites is the Boston Globe:
When you get there, if you're on your desktop and can change the size of your browser window, slowly make it smaller and watch how it changes the size and relationships of the content on the page as the window gets smaller or larger again as you open the window back up.
So the take away here should be that if you have a site (or want to have one) that's used for commercial purposes, it's will be worth your while to consider who your clientele is and what kind of devices they may be using to visit your site. If you're uisng Google Analytics to measure your site's performance, you will be able to see how many people are using their phones and tablets to view your site, which can give you the raw data you may need to make a design/redesign decision.