By default when you open an email conversation all the replies made is marked as read. However many a time you might have time to just read few of them and feel reading rest of them later. Mark Unread from here is a Google Laps feature for Gmail which allows you to partly read conversation and make rest of it as unread.
This is an extremely feature useful if you get loads of email and want to track them partly as you get time. You will have to enable this by going to Settings > Lab in your Gmail Account.