Don't get what happened when we declared the enum......

When you declare an enum like that, you're basically creating a type (so you can make functions and methods take only that type, for example), and then you're defining a list of possible values of that type. Say you have a class to represent a car, and you only want a car to be a specific set of colors. You could define an enum like this:

typedef NS_ENUM(NSInteger, MHCarColor){
    MHCarColorBlack,
    MHCarColorWhite,
    MHCarColorSilver,
    MHCarColorBlue,
    MHCarColorRed
};

and then define your Car class's initializer to take MHCarColor. That way you could do something like this:

[[MHCar alloc] initWithColor:MHCarColorBlack]; //or MHCarColorWhite, MHCarColorSilver, MHCarColorBlue, or MHCarColorRed

That initializer will only be able to take those specific values, so enums are a great way to define options for a type where there is a finite number of possible values

Also, that above example uses NS_ENUM, which is the newer, recommended method of defining enums, which you can read about on the NSHipster blog (which is a generally amazing place to learn about cool things in iOS programming)

If you wanna know what's going on under the hood, you're basically defining a bunch of integer constants that are generally entirely ceremonial, but can be whatever you want. Doing this:

typedef NS_ENUM(NSInteger, MHCarColor){
    MHCarColorBlack,
    MHCarColorWhite,
    MHCarColorSilver,
    MHCarColorBlue,
    MHCarColorRed = 100
};

is almost entirely equivalent to doing this:

typedef NSInteger MHCarColor;
static const MHCarColor MHCarColorBlack = 0;
static const MHCarColor MHCarColorWhite = 1;
static const MHCarColor MHCarColorSilver = 2;
static const MHCarColor MHCarColorBlue = 3;
static const MHCarColor MHCarColorRed = 100;

The enum method is generally accepted to be better, however, because it constrains the type (in this case, MHCarColor) to only have a certain number of possible values, it's a lot less typing, and it also works a lot better with Swift