What is #ifndef, #define in the header file?

This is Include guides

If the header file is included, Check if unique value is defined, such as HEADERFILE_H If it is not defined, define it and move on to the next code.

If the header file is re-included, In the first line of ifndef, fail to create the same effect as the inclusion of empty files. By doing this, identifier such as type, enum, and global variables are prevented from being defined several times.