Anyone knows what does advapi stand for?

It stands for Advanced Windows 32 Base API as it can be read on clicking with secondary (right) pointing device (mouse) button on file %SystemRoot%\System32\advapi32.dll, clicking with primary (left) pointing device (mouse) button on context menu item Properties and selecting next the tab Details with the File description showing this information.

Found on Microsoft documentation page How RPC Works:

Advapi32.dll is an advanced Windows 32 base API DLL file; it is an API services library that supports security and registry calls.