Within Teamwork, you can collapse a task list to hide the individual tasks it contains.
Collapsing and expanding task lists can be done individually per task list or for all task lists at once.
All task lists
To expand or collapse all task lists at once, click the options button (three dots) in the top right the project's Table or List area and select Collapse/Expand All Task Lists from the dropdown.
By clicking the arrow to the left of a task list name, you can expand or collapse that individual task list.
Note: This is an individual setting for each person viewing a project. Expanding or collapsing task lists will not affect the view for other project members.
For more information, see: Collapsing and Expanding Subtasks