Simply, It is a hit or request of which the data being sent to adobe servers.
Here is the detailed information on server call from AA documentation:
A server call, also known as a "hit" or an "image request", is an instance in which data is sent to Adobe servers to process. The most common type of server call is a page view. A page view occurs where a visitor views a page on your website and a server call is generated to Adobe, where information is collected, processed, and then included in your report metrics.
There are other types of server calls, including exit links and file downloads, where data is sent to Adobe to process, but they are not recorded as a new page view. Even "excluded" page views (excluded from your reports by an IP address range you configure, for example) are server calls because they are received and processed by Adobe but never show up in your reports.
Primary Server Call: Requests received directly from website visitor browsers or the Data Insertion API. Includes Primary Hits (Page Views), Primary Custom Events, Primary Download Events, and Primary Exit Events.
Secondary Server Call: Copies of primary server calls created by multi-suite tags or copied/moved by a VISTA rule. If a secondary server call has been moved (not copied) to a different report suite by a VISTA rule, the accumulated secondary calls are deducted from the primary server calls.
Mobile Primary Server Call
Requests received directly from one of the Mobile SDKs. Include trackAction, trackState, trackApp Crashes, trackActionFromBackground, trackLocation, trackBeacon, trackPushMessageClickThrough, trackTimedActionBacklog, trackLifetimeValueIncrease.
Mobile Secondary Server Call
Copies of primary server calls created by multi-suite tags or copied/moved by a VISTA rule. If a secondary server call has been moved (not copied) to a different report suite by a VISTA rule, the accumulated secondary calls are deducted from the primary server calls.
Note: If your company is contractually entitled to only Mobile Server Calls (Primary or Secondary), both your web and mobile-specific usage will count against your Mobile-specific commitment.
https://experienceleague.adobe.com/docs/analytics/admin/admin-tools/server-call-usage/overage-overvi...
