This would be the multi-select feature.

In your task list, tap the menu bars at the bottom right and choose multi-select.  Then tap each task you want to group together.

When you have made your selections, tap the menu bars at the bottom right again and choose from the options available for your group.  

If you change your mind about making any adjustments and want to get out of multi-select mode, tap the checkmark icon at the top left of that screen.