Voucher or bonus codes can be used to acquire certain in-game benefits. Codes are given at random times from bigpoint through their newsletter, promotional videos (twitch and youtube), facebook and other promotional websites. To redeem a code, open the shop (default key T), then click the second tab "payment" and select "Vouchers" from the menu. There you cen enter the code in the empty field box and click the Redeem Code button underneath.
You can also redeem codes in Browser at DSO Home Page (see the second picture below).
Codes generally have an expiry date, so the following codes may not work for you.
(New Codes can be found in the MediaWiki:Community-corner)