What is WordPress Customizer?

Wordpress offers a lot of customisation like page titles, icons, tagline, colors including theme options like header, sidebar, blocks. So, to use this Wordpress have added a panel which is known as a customiser.
Customiser can be accessed at Appearance > customiser or from admin menubar on top.
After opening customiser, you can change available options from basic as well theme options.

WordPress Customizer is a built-in tool in WordPress that allows users to customize various aspects of their WordPress site, such as the site's layout, color scheme, and widgets, among other things. It is accessible from the WordPress admin dashboard and allows users to preview changes to their site before publishing them. The Customizer provides a user-friendly interface that allows users to make changes to their site without the need to know any code. It is an important tool for users who want to customize the look and feel of their WordPress site without the need to write any code.