To redeem a voucher, you create a redemption object. Increments redemption counter and updates history of the voucher. This method is accessible through public keys which you can use in client side apps - mobile and web browser apps.
The client-side redemption works pretty the same as the regular voucher redemption. The difference lies in the authorization. For the client-side you can use client-side keys.
By default this feature is disabled. If you want to use it, you will need to enable function explicitly in Project Settings.
Be careful if you want to include the voucher redemption functionality directly at your client side (website or mobile app). In such configuration there is a chance that discounts can be modified before being sent to the server.