Planner is an Office 365 service that lets you generate boards containing all your team's tasks and assignments.
To add a Planner tab in Teams, just click Add a tab next to your tabs at the top of a channel. Select Planner, then do one of the following:
Click Create a new plan and type a name for the new Planner board.
Click Use an existing plan and select an existing Planner board from the menu.
Once you've created your Planner tab, you can assign different tasks to other people in your team and add links to individual tasks. To start a conversation about your Planner board where you can @mention teammates, just click Show tab conversation in the top right corner of your tab. That chat also appears as its own thread in the channel containing your Planner tab.
You might notice that some Planner features are missing from your Planner tab. For now, click Go to website in the top right corner of the tab to open your current board in the Planner web app, where you'll have access to all the features.