A single thread conversation is a feature offered by many email providers that groups together emails with the same subject line. This means all replies to an email are kept in the same place. In some situations it also applies to forwarded emails.
Without single thread conversations, emails are shown in an inbox in the order in which they are received. This means the replies arenot directly connected to the original email, and each reply is treated as a separate entity, or a separate email.
Using a view that utilizes single thread conversations makes it easier to track all replies and forwards to an email because they are kept in one place. It typically works by displaying the unread replies and forwards, with the original email along withreplies and forwards that have already been read being displayed in a uncollapsed format. This allows the user to see in one place the full conversation relating to the email, with priority given to the unread parts. The read parts are usually easily accessed by expanding all of the messages. They are all displayed in a stack-like format, typically in the order in which they are received.
Popular email services like Yahoo and Gmail offer this type of message view. The benefits are that email conversations are easier to follow, and replying to all of the conversation, or to certain parts of the conversation, is easier.