Office 365 (Outlook) - Using Tasks / Microsoft To-Do
A task is an item that you create that you want to track until it's completed. Tasks can be created from an email that you've flagged for follow-up as an actionable item, or they can be to-do list entries that you add manually. Use tasks in Outlook on the web to create, edit, categorize, and manage tasks.
For the most current information on Tasks within Outlook, please see the following Microsoft help pages:
Outlook client: https://support.microsoft.com/en-us/office/create-tasks-and-a-to-do-list-in-outlook-6de6ee52-751b-4405-b389-850572b15306