Task dependencies are the relationships between tasks that show what must be done before a given task and what will be done after the task is completed. You can have dependencies between tasks in the same project and between tasks in different projects.
Task dependencies are not enabled by default, you have to enable under the projects settings
This will enable the dependecies and add the "Link" option to the task side menu. Click on the menu to start linking tasks. Linking tasks creates dependencies between them.