An instance of a class is a real world item that fulfils the criteria of the intension of the class. Note, that the number of instances declared for a class in an information system is typically less than the total in the real world. For example, you are an instance of Person, but you are not mentioned in all information systems describing Persons.
The painting known as the “The Mona Lisa” is an instance of the class Man Made Object.
An instance of a property is a factual relation between an instance of the domain and an instance of the range of the property that matches the criteria of the intension of the property.
“The Louvre is current owner of The Mona Lisa” is an instance of the property “is current owner of”.