How to redeem
Go to Settings → Billing and enter the code under “Have an access code?”. The plan starts right away and runs for the number of days the code grants. Each code can be redeemed once per account.
What to expect
Notes plans from a code apply to your personal workspace; agency plans are managed by the agency owner. If you already pay for a plan on the same product, the code wouldn't add anything, so it isn't applied.
When the code's time ends you move back to the Free plan. Nothing is deleted and you keep access to your records.