Annotations are the comment-style markup that tells the SharePoint theming engine how to theme the properties in a CSS file. In SharePoint 2013, annotations can be used to change the color, font, or the background image.
The types of annotations available in SharePoint 2013 are as follows:
1.ReplaceColor
2.ReplaceFont
3.ReplaceBGImage
4.RecolorImage
Annotations are really useful in your custom CSS file when you want to override the default CSS file core15.css.