When you browse to the project, you will be prompted for a user id and password as well as a comment. The comment you enter will be used during the whole session until you either change it or log out. It will be stored with any change to the project's data you make.
You can also login without a session comment, e.g. if you want to read only or if you want to provide a specific comment for every change.
If you want to use a comment over multiple sessions, you can tick "remember comment".
The session will expire automatically after 30 minutes (default) without activity. The session timeout can be changed by any administrator here: Password Settings
If you remove the comment you will be prompted to enter a comment whenever you want so save a change.