Forwarded emails, in this context, are emails that are sent to one email address but then automatically forwarded to another email address. This is different from a recipient of an email manually forwarding this to someone else.
Some people see forwards as a big problem when looking at DMARC. Forwards are, however, a very small percentage of overall emails so this is not a large issue. There is also work underway in the standards bodies to address this (See ARC below). Valimail is following this closely and has contributed open source code and is an editor on the proposed ARC standard.
The classic example of forwarding is as follows:
A person has an alumni email address from a school they attended that they use as their 'public' email address (The address that they tell people to email them at). In this case however, the person has configured their alumni account to automatically send all of these emails onto the person's Gmail (or other) account.
Related article:
Problems You May See Due to Forwarders and What To Do