Think of it this way: instructions is a function defined internally to the makeCake function, only it has no body to it, just a declaration. The trailing closure supplies that missing function body. And how would you call a function named instructions? Like so: instructions().

So let's say you've got something like this:

func instructions() {
    print("Mix egg and flour")
}

func makeCake() {
    print("Wash hands")
    print("Collect ingredients")
    instructions()  //call the instructions function defined above
    print("Here's your cake!")
}

But instead of hardcoding the instructions function, you can turn it into a parameter to the makeCake function:

func makeCake(instructions: () -> Void) {
    print("Wash hands")
    print("Collect ingredients")
    instructions()
      //since instructions is now a closure, you can call it by adding () after the name
    print("Here's your cake!")
}

which you can then call like this:

makeCake(instructions: instructions)
  //passing the name of the function makeCake will call internally

Or you can get rid of the explicit instructions function and supply a closure instead;

makeCake(instructions: {
    print("Mix egg and flour")
})

Which, using trailing closure syntax, can be shortened to:

makeCake {
    print("Mix egg and flour")
}

In order for the closure makeCake to work, instructions must be called?

Yes, because otherwise the closure you are passing in as a parameter will not be given the chance to do its thing.