The addObject: method is used when dealing with NSMutableArray. It is used to add items onto the stack of the array. A simpler example of this is to imagine a stack of paper. When calling the addObject method, it is essentially the same as a person adding a piece of paper onto the stack of paper.

NSMutableArray *mArray = [[NSMutableArray alloc] init];
 
//adding object
[mArray addObject:@"Hello"];
 
// displaying the contents of the array into the console
for(id obj in mArray) {
   NSLog("%@", mArray);
}