Null-terminated string in C++

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C++

i was reading a sample code in a c++ book and it had the code:

#include <iostream>
#include <string>
#include <string.h>

using namespace std;

int main()
{
   cout << "enter a line of text: ";
   string userinput;
   getline (cin, userinput);

   char copyinput[20] = { '\0' };
   if (userinput.length() < 20)
   {
      strcpy(copyinput, userinput.c_str())
      cout << "this does not exceed the buffer";
   }
   else 
   {
      cout << "this exceeds the buffer!";
   }

   return 0;
}

the thing i don't understand is why they put a '\0' at the when initialising the 20th element of the array, copyinput. I tried the program without adding the '\0', and it still worked, so i dont know why they put it there. can you please help me??

What I have tried:

i dont know what to search up about this