To redeem vouchers you must go to the "Vouchers" section of your app. Click "Add voucher code", write it down and click "Confirm". Once done, the voucher will be added to your voucher jar.
Each one of those will have its own features and requirements, as well as its own expiration date. Remember to use it by then!
Know more about how to use vouchers to purchase tickets.
In order to redeem a voucher, the latest version of the app must be installed in the device. Voucher redemption may fail due to several reasons. Most common reasons are stated here.