How does add activity action work inside a deal card?

Activity is your calendar, where you can schedule meetings and other activities.  You can include other agents or team members.

Activity Vs Task

Activity is when you want to create an activity with the contact/client or prospect and the contact/client or prospect will receive an email notification-Activity is your calendar.

A Task is when you want to create a Task for yourself or a team member to do something/remind them of something regarding a client/prospect which has the option to add to Calendar.

Inside the deal card, the column on the left shows all of the actionable items.  Click on Activity at the 2nd row of this section.

From the pop up screen, you can schedule your activity

You can see a history of all the activities assigned in the middle of the screen.

If you click on the upcoming or past tab, to view all upcoming or completed Actions 

And the tasks, you can add tasks that are to be performed. the added tasks will be visible in your calendar.

The calendar events will also show up all the tasks on your Dashboard.  This way, you can make sure to follow up.