The cache manifest is a part of the deprecated HTML5 application cache (AppCache) feature, which is intended to provide users with the ability to access a web application even without a network connection.
Web applications consists of a list of web addresses. These can be HTML, CSS, JavaScript, images or any other source that is required for a web application to be rendered. These addresses or URLs can be copied onto a manifest file, which can be updated regularly by the author of the web application, indicating any new web addresses that are added or deleted. When connecting to a network for the first time a web browser will read the HTML5 manifest file, download the resources given and store them locally. Then, in the absence of a network connection, the web browser will shift to the local copies instead and render the web application offline.
(Source: wikipedia.org/Cache_manifest_in_HTML5)