By default, each task is visible to all project members. However, you have the option of marking a task as confidential.
Task > What tab of the task detail: click on the padlock icon to the right of "Confidential task".
When confidentiality is active, the icon turns blue . In the What view, a small padlock icon appears on the task.
To turn confidentiality off, click the "Confidential task" toggle button again in the What tab of the task details panel.