Now I'm not sure if I'm being unclear, but read through the ARC docs, or check through the SO tag.
To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.
To make this possible, whenever you assign a class instance to a property, constant, or variable, that property, constant, or variable makes a strong reference to the instance. The reference is called a “strong“ reference because it keeps a firm hold on that instance, and does not allow it to be deallocated for as long as that strong reference remains.
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Weak and unowned references enable one instance in a reference cycle to refer to the other instance without keeping a strong hold on it. The instances can then refer to each other without creating a strong reference cycle.
Use a weak reference whenever it is valid for that reference to become nil at some point during its lifetime. Conversely, use an unowned reference when you know that the reference will never be nil once it has been set during initialization.
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Weak References
A weak reference is a reference that does not keep a strong hold on the instance it refers to, and so does not stop ARC from disposing of the referenced instance. This behavior prevents the reference from becoming part of a strong reference cycle. You indicate a weak reference by placing the weak keyword before a property or variable declaration.
Use a weak reference to avoid reference cycles whenever it is possible for that reference to have “no value” at some point in its life. If the reference will always have a value, use an unowned reference instead, as described in Unowned References. In the Apartment example above, it is appropriate for an apartment to be able to have “no tenant” at some point in its lifetime, and so a weak reference is an appropriate way to break the reference cycle in this case.