Resolution is This is the "size" of your display measured by pixels. Nowadays, 2016, a common resolution is "1366 x 768" meaning that the display is shown using 1366 pixels horizontally and 768 pixels vertically. If this is your screen "native resolution", ie, the physical number of pixels in your monitor, you should select this resolution for the clearest image possible. There are many ways to check your screen resolution, but a fast way is to use a online service like what-is-my.com.
This web site will show also your browser window resolution and your browser window width, among other parameters. As it is plataform independent, It is compatible with Android devices, windows notebooks and desktops, as well as Apple products, like Ipad and Iphone.