FFD means fee-free dollar. As you earn fee-free dollars, the Plastiq fee is lowered for payments you apply them to. For example, if you have 1,000 FFDs, that is $1,000 worth of bills or invoices you can pay without the Plastiq fee.
NOTE: Based on the example above, any amount over the $1,000 payment, is subject to the standard Plastiq fee.
If you are interested in earning FFDs, check out this article on promotions and referrals.