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> What does `ptr()` mean in this code?

An expression like `ptr` followed by parentheses as in `ptr()` is a function call.  In your example, `ptr` is a variable of type "pointer to function" because of the declaration `void (*foo)()`.  That is, `ptr()` will call the function that the variable `ptr` points to.  And, due to the assignment `ptr = PrintHello`, the variable `ptr` contains the pointer to the function `PrintHello`.

For those not familiar with function pointers, the syntax appears strange, and some magic seems to happen.  To make it a bit easier to understand, I have used your example code and added some alternative ways to write the assignment and function call.

```
int main ()
{
    void (*ptr)(void); // ptr is a pointer to a function that
                       // returns void (first void in the line)
                       // and takes no arguments "(void)"

    ptr = PrintHello;  // PrintHello is implicitly converted to
                       // pointer to function - you could also write:
    ptr = &PrintHello; // Some people prefer writing it like this to
                       // make it clear that a function pointer is used

    ptr();             // Call the function.  Could also be written as:
    (*ptr)();          // Some people prefer writing it like this to
                       // make it clear that a function pointer is used

    // Ignore the below part if you are not really interested in some
    // background about how the C standard defines the stuff - which
    // may be surprising, because it appears somehow upside-down...
    PrintHello();      // Normal way of writing a function call, but:
    (&PrintHello)();   // This is what the compiler makes of it.
                       // See C standard 6.5.2.2#1 with footnote and
                       // 6.3.2.1.#4
}
```

On a side note, to remember the function pointer declaration syntax, I always think about how the function call would look like.  I come up with the expression `(*ptr)()`, because I think: "first dereference the function pointer to get the function, afterwards call it with the arguments".  Then I am almost done, because the syntax for the call is similar to the syntax of the declaration.