Quote from RaceDoctor :

Someone else will buy me an S2 licance. In the buying page there's a "S2 voucher" under the title "Buy LFS for some else". So what's that? And how it works?

Could someone explain to me?

Take a look at this page:
(All the info you need , I think)

Quote from https://www.liveforspeed.net/?page=shopinfo :

"There is also the option to buy a license for someone else. This is done via a so called 'Voucher', which is a special code that can be entered on our website, to upgrade an account to S2 licensed. So when buying a license for someone else, you buy a voucher, which is a code that the other person can use to upgrade his account here on the Live for Speed website."

url: https://www.liveforspeed.net/?page=shopinfo
Hope this helps you and welcome to the world of LFS

Think of it as a 'Gift Certificate'

So the person buying for you will get a code, which he will send to you (per email or any other way) and you will enter this code on the account details page on www.liveforspeed.net, click and you are S2 licensed