Each of your chats has its own security code used to verify that your calls and the messages you send to that chat are end-to-end encrypted. This code can be found in the contact info screen, both as a QR code and a 60-digit number. These codes are unique to each chat and can be compared between people in each chat to verify that the messages you send to the chat are end-to-end encrypted. Security codes are just visible versions of the special key shared between you - and don't worry, it's not the actual key itself, that's always kept secret.
At times, the security codes used in end-to-end encryption may change. This is likely because someone reinstalled WhatsApp or switched phones.
Important: You and your contacts must be using the latest available version of WhatsApp for end-to-end encryption to be activated.
To receive notifications when security codes change:
Note: Your calls and the messages you send are end-to-end encrypted regardless of this setting, when possible.
Keep in mind that you can always verify that a contact's security code is legitimate. Learn how in this FAQ article.