Free quota is extra upload quota on top of your plan's annual quota. You earn it by finishing guided setup or through the referral program, and it's yours until you use it.
How to earn it
Guided setup: new accounts earn 5 GB for each setup step they finish, 25 GB in all. The steps are: create a project, upload a video, create a presentation, publish it, and share it. Guided setup is offered once, to accounts created since it launched.
Referrals: see How does the MediaZilla referral program work?
Both add to the same balance.
How it's used
Every upload uses your free quota first, then your plan's quota.
It doesn't reset each year the way your plan's quota does. It stays until you use it.
When you upgrade, you keep it on top of your new plan.
If you cancel your membership, any free quota you haven't used is removed.
Where to see it
Go to Company Settings > Billing. Under Upload Quota, Free quota shows how much you've earned and how much is left. The Upgrade Plan page also shows how much free quota you have.