Few years back internet users had monitors with almost same screen resolution. Web designers were used to design websites for same screen resolutions and their task was easy. But now the scenario is different. Now users have different devices to surf internet. The devices may be desktops, laptops, tablets or smartphones and all these devices have different screen resolutions. Web designers now need to make their designs compatible with most of the possible screen resolutions. It is not feasible to test the designs in all of these devices but a web designer can re-size the browser as per the screen size of particular device.
There are many online tools, Firefox add-ons and Chrome extensions available to test your designs in different screen sizes. These tools will make your task easier to check how your website looks in different resolutions.