macOS Can Someone Explain This Code?

Same name? It's defining two IBOutlets. Both are of type NSTextField *. One is named pathField. The other is named outputField.

You also generally don't send IBActions. IBActions are sent to you.

It sounds like there is a fair bit of confusion here.

IBOutlet is just a macro that Xcode/Interface Builder uses to determine which variables can act as outlets.

"IB" stands for "Interface Builder". What this is for is to allow you to connect a NIB object to a program object so that when the program starts up, the correct outlet object will be properly connected to the controller object that needs to have access to it. For instance, if you need to change the name of a specific button, you need to have a reference (outlet pointer) to that specific button.

Apart from Interface Builder connections, "IBOutlet" is an otherwise meaningless specifier, and "IBAction" is equivalent to "void".

As you noticed and pointed out, pathField and outputField are pointers to NSTextField objects.

But where are those objects coming from? You might have also noticed that nowhere in your code do you alloc and init an NSTextField, they are magically just there, able to be used.

NIB files are files that contain objects, already created/init'ed and ready to be used. When the NIB file is loaded, so are those objects. IBOutlets tell the compiler that those pointers will be pointing to objects in a NIB file. The connections need to be made in interface builder so the compiler will know which object in the NIB file the pointer points to.

So without the IBOutlet, those pointers would point to nothing until your code created an object and assigned the pointers to point to the new object. With the IBOutlet, those pointers point to objects in the NIB file.

Also, IBActions are unrelated and not required for IBOutlets. ABActions work in the reverse. There are pointers in the objects in the NIB file. IBActions are methods that those pointers can point to. So, if an object in the NIB file needs to send a message to an object in your code, an IBAction will allow that connection to be made. The most common would be if a button is clicked, that button object (in the NIB file) sends a message to an IBAction. The pointer in the NIB file needs to point to that IBAction, and that is done by the connection in IB.

Hope that makes sense.