On this page of the guide to Back 4 Blood, you will learn what supply lines are and what you can unlock with them.
Supply lines work like a battle pass. When playing the game, you receive a special currency, the supply points. Use them to unlock supply lines.
Initially, you will be able to unlock only one supply line, but if you complete the entire line and get all the rewards from it, you will unlock many more.
Each supply line contains many different rewards. In the main one, you can unlock unique cards that you can include in your deck, but also new sprays, banners or badges, that is, items for customizing your profile. The more progress you make in a given supply line, the more supply points you will need to spend. Remember that if you upgrade one supply line, for example, you don't have to finish it to start the next one. You can unlock rewards in several supply lines at once.