Progress bars are used as a feedback mechanism for system tasks. As long as the process is running, the progress bar grows continuously from 0% to 100%. Use it for system processes.
The progress bar grows in primary color, and the label changes to reflect the completion percentage.
The warning state indicates that something happened and interrupted the process.
The finished state indicates that the process is completed successfully when the label changes to the success icon.
Dos and Don'ts
Use it for processes such as a file upload.
Never use it to communicate the progress of a user in a certain set of actions. For that you need a multi step form type.