When a book says that it is all about wordpress theming, what does it mean?
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In short, it refers to WordPress theme development.
Theme development, i.e. the creating, designing and developing of a theme that will be available for the WordPress CMS/platform. Themes can be accessed from the Appearance > Themes menu, and have settled on two niches, free and premium themes.
The book you mention, if about 'wordpress theming', will either guide you through, or point-out the relevant aspects of developing/customizing a theme for WordPress.
EDIT: Just to clarify, the verb 'theming' itself can be translated to skinning, customizing, adapting, etc. Hence, WordPress theming could also just refer to customizing rather subtle aspects and elements of an existing theme, such as widgets, navigation menus, and so on.