A dictionary stores key-value pairs. Each key maps to a value. Think of a real dictionary: you look up a word (the key) to find its definition (the value). Python dictionaries work the same way. python phone_book = {"Alice": "555-1234", "Bob": "555-5678"} Keys are unique.
You can't have two entries for "Alice". Values can repeat. Lookup is fast. Finding a value by its key takes the same time whether you have entries or million.