C++ std::strings are "counted" strings - i.e., their length is stored as an integer, and they can contain any character. When you replace the third character with a nothing special happens - it's printed as if it was any other character (in particular, your console simply ignores it).
In the last line, instead, you are printing a C string, whose end is determined by the first that is found. In such a case, cout goes on printing characters until it finds a , which, in your case, is after the third h.
is the NULL character, you can find it in your ASCII table , it has the value 0. It is used to determinate the end of C-style strings. However, C++ class std::string stores its size as an integer, and thus does not rely on it