Holding a licence for a particular good increases your income from that good, by a fixed amount per transported ton.
In the lottery you can win a Licence Voucher, which lets you buy one licence for free.
When you buy a licence with gold or a voucher, you can pick any licence that's available, you don't have to wait for the right time when that particular auction starts.
You can upgrade the Licence Trade to increase the number of licences you can hold at the same time. You can delete a licence you already own, if you want to make room for a new licence.
Every fifteen minutes a new licence auction starts. The goods for auctions are selected randomly, and announced 27 hours in advance. Each auction lasts 14 minutes, followed by a one minute waiting time before the next auction starts. During the auction you can see which players have bid on the current licence. For one minute after the end of an auction, you can see who have bid how much.