I've installed QGIS Server and it works fine. But in /usr/lib/cgi-bin/ folder I've found 2 additional files -
admin.sld. What is the purpose of these file? I've never used them.
The admin.sld is a file that holds information about the layers and styles of your WMS. SLD stands for Styled Layer Descriptor and is a OGC Standard. Here you have the specification: http://www.opengeospatial.org/standards/sld
You can open it and modify the content to change the symbolization. Thus it is an alternative way of defining your WMS content (other way is to save your qgis project file on the server)
The metadat file hold information about the owner of the WMS as well as contact information (if you have defined them). These are used in the get capabilities request to give users information about the WMS owner.