Headers style.css

This is a special file - it is how WordPress identifies the theme.

When you go to the Themes section in the admin panel, WordPress scans all folders in the themes directory for the presence of the file style.css. Those folders that contain style.css with the header Theme Name: are identified as theme folders.

The theme name and other data are taken from the file style.css. Such data is read from the information at the beginning of the file (they are located in comments):

/**
 * Theme Name: My first theme
 */

Complete list of parameters for the style.css file

Along with the Theme Name field, you can specify other fields.

Theme Name(required)
The name of the theme. This is a required field! It is enough to specify this field for WordPress to identify the theme. Other fields are not required but desirable.
Template
The name of the parent theme. Example: Twenty Seventeen.
Description(required for the WP repository)
A brief description of the theme.
Theme URI
URL of the page where you can find more information about the theme.
Author(required for the WP repository)
The name of the person or organization that developed the theme. It is recommended to specify the username on wordpress.org.
Author URI
The URL of the author or organization.
Version(required for the WP repository)
The version of the theme, in the format X.X or X.X.X.
License(required for the WP repository)
The license of the theme.
License URI(required for the WP repository)
The URL of the license.
Text Domain(required for the WP repository)
A string used as textdomain when translating.
Domain Path
The path to the MO translation file relative to the theme folder. Used so WordPress knows where to find the translation when the theme is disabled. Default is /languages.
Tags
Words or phrases, separated by commas, that allow users to find the theme using tag filters. A complete list of tags is provided in the theme review guide.

required for the WP repository means that the field is required if you plan to submit the theme to the WordPress theme directory (repository).

Example of a complete header for the style.css file

/**
 * Theme Name:  Theme Name
 * Theme URI:   URL of the theme in the WordPress directory or elsewhere. Example: http://wordpress.org/themes/twentythirteen
 * Author:      The name of the theme author
 * Author URI:  The author's URL. Example: http://mysite.org/
 * Description: A short description of the theme.
 * License:     License. Example: GNU General Public License v2 or later
 * License URI: URL for the license. Example: http://www.gnu.org/licenses/gpl-2.0.html
 * Tags:        Theme tags that can be used to find the theme in the WordPress directory. For example: black, brown, orange, tan, white, yellow
 * Text Domain: The translation domain for the theme. Needed to translate the description specified in Description. For example: twentythirteen
 * Version:     The version of the theme. Example: 1.0
 */