Web app manifests are part of a collection of web technologies called progressive web apps (PWAs), which are websites that can be installed to a device’s homescreen without an app store. Unlike regular web apps with simple homescreen links or bookmarks, PWAs can be downloaded in advance and can work offline, as well as use regular Web APIs.

The web app manifest provides information about a web application in a JSON text file, necessary for the web app to be downloaded and be presented to the user similarly to a native app (e.g., be installed on the homescreen of a device, providing users with quicker access and a richer experience). PWA manifests include its name, author, icon(s), version, description, and list of all the necessary resources (among other things).

https://developer.mozilla.org/en-US/docs/Web/Manifest
The name member is a string that represents the name of the web application as it is usually displayed to the user (e.g., amongst a list of other applications, or as a label for an icon). name is directionality-capable, which means it can be displayed left-to-right or right-to-left based on the values of the dir and lang manifest members.
The short_name member is a string that represents the name of the web application displayed to the user if there is not enough space to display name (e.g., as a label for an icon on the phone home screen). short_name is directionality-capable, which means it can be displayed left-to-right or right-to-left based on the value of the dir and lang manifest members.
The description member is a string in which developers can explain what the application does. description is directionality-capable, which means it can be displayed left to right or right to left based on the values of the dir and lang manifest members.
The theme_color member is a string that defines the default theme color for the application. This sometimes affects how the OS displays the site (e.g., on Android's task switcher, the theme color surrounds the site).
The background_color member defines a placeholder background color for the application page to display before its stylesheet is loaded. This value is used by the user agent to draw the background color of a shortcut when the manifest is available before the stylesheet has loaded.
The display member is a string that determines the developers’ preferred display mode for the website. The display mode changes how much of browser UI is shown to the user and can range from browser (when the full browser window is shown) to fullscreen (when the app is full-screened).
The orientation member defines the default orientation for all the website's top-level browsing contexts.
The scope member is a string that defines the navigation scope of this web application's application context. It restricts what web pages can be viewed while the manifest is applied. If the user navigates outside the scope, it reverts to a normal web page inside a browser tab or window.
The start_url member is a string that represents the start URL of the web application — the preferred URL that should be loaded when the user launches the web application (e.g., when the user taps on the web application's icon from a device's application menu or homescreen).
The categories member is an array of strings defining the names of categories that the application supposedly belongs to.
The base direction in which to display direction-capable members of the manifest. Together with the lang member, it helps to correctly display right-to-left languages.
The lang member is a string containing a single language tag. It specifies the primary language for the values of the manifest's directionality-capable members, and together with dir determines their directionality.
Recommend 256*256 png format and Size limit 3Mb
If you do not want to place icon files at the root of your web site