Explanation of the get-put principle

I have read O'Reilly book, in that I came to know this get-put principle.

• Use an extends wildcard when you only get values out of a structure.
• Use a super wildcard when you only put values into a structure.
• And don't use a wildcard when you both want to get and put from/to a structure.

Exceptions are:

• You cannot put anything into a type declared with an extends wildcard except for the value null, which belongs to every reference type.

• You cannot get anything out from a type declared with an super wildcard except for a value of type Object, which is a super type of every reference type.

Can anyone help me to explore this rule at depth? If possible, please put them hierarchical manner.