If you selected a language in which has more than one country associated with it (true for Chinese, English, French, and many other languages), you can specify the country here. If you choose Other at the bottom of the list, you will be presented with a list of all countries, grouped by continent. This selection will be used later in the installation process to pick the default timezone and a Debian mirror appropriate for your geographic location. If the defaults proposed by the installer are not suitable, you can make a different choice. The selected country, together with the selected language, may also affect locale settings for your new Debian system.