Explanation

number = phone_dir["Martin Eden"]
This code uses the square brackets notation to access the value associated with the key "Martin Eden" in the phone_dir dictionary. The value is then assigned to the variable number. A dictionary is a data structure that stores key-value pairs, where each key is unique and can be used to retrieve its corresponding value. You can find more information about dictionaries in Python in the following references:
[Python Dictionaries - W3Schools]
[Python Dictionary (With Examples) - Programiz]
[5.5. Dictionaries - How to Think Like a Computer Scientist ...]