AndroidManifest.xml is a required file for every application. It sits in the root folder for an application, and describes global values for your package, including the application components (activities, services, etc) that the package exposes and the implementation classes for each component, what kind of data each can handle, and where they can be launched.

An important aspect of this file are the intent filters that it includes. These filters describe where and when that activity can be started. When an activity (or the operating system) wants to perform an action such as open a Web page or open a contact picker screen, it creates an Intent object. This object can hold several descriptors describing what you want to do, what data you want to do it to, the type of data, and other bits of information. Android compares the information in an Intent object with the intent filter exposed by every application and finds the activity most appropriate to handle the data or action specified by the caller. More details on intents is given in the Intent reference page.

Besides declaring your application's Activities, Content Providers, Services, and Intent Receivers, you can also specify permissions and instrumentation (security control and testing) in AndroidManifest.xml ...

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The semantics of categories is the opposite of actions -- an Intent includes the categories that it requires, all of which must be included in the filter in order to match. In other words, adding a category to the filter has no impact on matching unless that category is specified in the intent.

There you find links/descriptions to things like the Category:

The semantics of categories is the opposite of actions -- an Intent includes the categories that it requires, all of which must be included in the filter in order to match. In other words, adding a category to the filter has no impact on matching unless that category is specified in the intent.

...
<action>
An Intent action that the component supports.
<category>
An Intent category that the component supports.
<type>
An Intent data MIME type that the component supports.
<scheme>
An Intent data URI scheme that the component supports.
<authority>
An Intent data URI authority that the component supports.
<path>
An Intent data URI path that the component supports.
...