inline makes defined function as a copy-paste code, i.e. it suggest to compiler that "You can replace this code, instead of calling it as a function.". Let me explain with an example. We code this program:

#include <iostream>

inline void print(string name) {
    std::cout << name << endl;
}

int main() {
   string myName = "Meysam";
   
   print(myName);
   
   return 0;
}

When you compile this code, your code suggest to your compiler that "It's a good idea if you replace print function with it's code, huum?" and based on situation, if compiler accepts this suggestion, code will converts to this:

#include <iostream>

inline void print(string name) {
    std::cout << name << endl;
}

int main() {
   string myName = "Meysam";
   
   std::cout << myName << endl;
   
   return 0;
}

and then compiled.

This optimision feature of C++ in many situations is a good solution instead of using macros. It will reduce overhead of program, but this also has some disadvantures that you can find them with search on google :D.

If any explaination is required, I'm here ;).