Do you want an explanation, a correction, or an example use (e.g., with fork)? A few quick notes about that line:

It prints the decimal value of x followed by a newline. Example:
int x = 42;
printf("Parent: %d\n", x);

Requirements:

include x should be defined and initialized and have a type matching %d (i.e., int). Passing the wrong type is undefined behavior.

Common pitfalls:

If x is uninitialized you get undefined behavior/garbage output.If x is a long or long long use %ld or %lld (or cast to int if appropriate).If x is unsigned use %u.If x is a pointer use %p and cast to void*.

If used around fork():

stdout is buffered; if you fork without flushing, you may see duplicated output. Call fflush(stdout) before fork or print to stderr (which is unbuffered) to avoid duplicate buffered text.
Example:
fflush(stdout);
pid_t pid = fork();
if (pid > 0) printf("Parent: %d\n", x);
else if (pid == 0) printf("Child: %d\n", x);

Tell me which of these you want (explanation, corrected code, fork example, or debugging a specific problem).