Other than the "/" role in the C++ #include statement

In many projects, Look at the embedded statements, such as #include <abcd/efg.h>.

I think this means "get me the abcd or efg header file" ... which means I don't know the role of the "/" in the enclosure.

Please give me a link or explanation of the document that tells me the rules for the header file enclosure.

I don't understand myself why I can't find the description of the preprocessor.