Workflow consists of process steps where a set of tasks can be defined for users to complete. These tasks can be added while designing a workflow.
On the process widget, go to "Show details". It will take you to the process detail page where you can add tasks.
When you add a task, specify name and you can mark this task as mandatory. Which means if this task is not completed, the process will not be marked as complete and the workflow will not move forward.
Next, you select the type of task for process.
Simple Task - You can provide the textual information here
Validation Task - Business rules can be added that are to be associated with the WF. If there are exceptions after rule execution, the task is considered as failed. If the rule passes, only then the task can be marked as complete.
File Upload Task - if a user is required to upload a file, you can mention the folder and the file name which will restrict the user to upload file with same name.
Note: File upload capability is only for CSV and Excel.