The SubID is a tag that you as a publisher can assign to your affiliate link. In your statistics you can see how often this tag has been called, i.e. how often that link has been clicked. With the transaction, the SubID is also shown. This way, as a publisher, you know which link generated the transaction.
The SubID is a very important tool for a publisher to measure which links generate the most sales. In addition, SubIDs are often used by cashback publishers to keep track of which member is responsible for which transaction. Keyword marketers often add the keyword they have purchased to the SubID. This way, they can keep track of which keyword generated the transaction. The SubID is only for the publisher; advertisers cannot see or use the SubID.
You can read more about the SubID for publishers in our Publisher FAQ.