What is a customizer?
You can find the WordPress customizer by clicking “Customize” in the toolbar, or under “Appearance” in the admin bar. The customizer is primarily used to determine the global look of one’s page. For example, the font and its sizes and colors, layout, responsiveness, button colors, header and footer, etc. However, the options in the customizer depend on the theme. So what you can edit in the customizer differs from theme to theme.
It is also in the customizer that you can select global logo, insert favicon, determine home page and add extra CSS. Its primary function is precisely to determine the global appearance of the website.