Unclear as to underscore requirement?

In the third part of this challenge, the assignment is to pick out an item from the array shoppingList and assign it to NSString shoppingCart.

self.shoppingCart= [_shoppingList objectAtIndex:2];

I only discovered this because I got close enough to this syntax that the editor itself told me what was missing. Is this challenge supposed to be a lesson in using the editor? I know underscore is used at times in methods, but that wasn't covered in ANY lesson up to this point. What the hell?

When do I use underscore? In the video, the closest usage,

self.funFactLabel.text = [facts objectAtIndex:1];

DOES NOT USE THIS!

#import "UIViewController.h"

@interface ViewController : UIViewController

@property (strong, nonatomic) NSString *shoppingCart;

@end

#import "ViewController.h"

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    // Add your code below!
}

@end