Discount codes give customers a percentage or fixed amount off at checkout. Create and manage them in Settings → Discounts.
Open Settings → Discounts and select New discount.
Add a name (e.g. "Early bird") and a code (e.g. EARLYBIRD).
Choose the type: Percentage or Fixed amount, and set the value.
Choose whether it applies to All events or Specific events.
Optionally limit the redeem-by date and the total number of redemptions.
To let customers enter a code at checkout, turn on Enable discount codes in the event's Pricing section. The total-redemptions limit applies across all customers, so it won't prevent a single customer from redeeming more than once.
Deleting a discount is permanent, but existing bookings that used the code aren't changed. And note manual discounts you apply on admin bookings aren't code redemptions - they won't count toward a code's usage figures.