What does '&' after class name mean?

#include <bits/stdc++.h>

int main () {
  std::string foo = "string_1";
  std::string bar = "string_2";
  std::vector<std::string> myvector;

  myvector.push_back (foo);
  myvector.push_back (std::move(bar));
  for (std::string x:myvector) 
    std::cout << x << '\n' ;
}

How's that code is diffrent when I exchange

  for (std::string x:myvector)

for?

  for (std::string& x:myvector)

I'm guessing there are a lot of places when I could find that, but I don't know what's the name of this measure, so I don't know what I should search for. Link to explanation will be enough if it's it's easier for you.