I didn't get this question, alloc means to allocate the memory and what about init it??

Not an Objective C expert, I'm much more experienced with Swift, but form what I understand alloc and init are both methods of NSObject and all Objective C classes extend from NSObject. Alloc returns a pointer to the memory location of the object. Notice the "*" that is always put after the type in an alloc call.

NSMutableDictionary * carDict = [[NSMutableDictionary alloc]initWithObjectsAndKeys: @"Honda",@"Make",@"Accord",@"Model",nil];

That's the pointer part. When alloc is run the program makes a blank copy of the object, assigns it a space in memory, and then returns a pointer of type id. The id type is, for lack of wanting to get to in depth here, a generic object pointer. The important thing to note about alloc, is that you can not use the it's return without calling init. Basically, all alloc has done is created you a blank object with no state, and in order to give that object state you call one of it's init methods. The init methods of an object are what actually return you the fully functional object (actually init returns a a pointer as well the, but this pointer now points to) . It's important to note that alloc and init may not always return a pointer to the same thing.