Refer to this article if your version is below 7.4.1

Common activities are activities that are not assigned to any project. These are created regardless of the project and can be standalone unlike earlier activities created under a specific project. It is optional to select a project for the common activities when filling the timesheets in common activities. To add common Activities below steps can be followed;

Step 1 - Select Time Tracking > More >Activity Info > Common Activities. 

Step 2 - Click the Add button.

Step 3 - Give a name to the activity and click Save.