A contact group in Microsoft Outlook is like a folder that holds a bunch of email addresses or contacts that you can send messages to all at once, instead of typing each person's email address separately. It's a way to organize your contacts and quickly communicate with multiple people without having to manually enter each person's email address every time.
Note: This is different than a Distribution List, which can be used by multiple people.
Please refer to the following Microsoft articles on creating contact groups:
Creating contact groups in the Windows Outlook client
Creating contact groups in the macOS Outlook client