Описание тега ibaction
IBAction is a type qualifier used by Interface Builder to synchronize with Xcode.It is available in iOS 2.0 and later. The tags [UIButton] should be used to ask about Apple's UIKit frameworks UIButton action. Use the related tags [ios], [UIButton] and [IBAction] for issues specific to those platforms.
IBAction is a macro defined to denote methods that can be referred to in Interface Builder.
As indicated in Apple's documentation, it is defined as void:
#define IBAction void
Relevant stackru question: IBOutlet and IBAction