On this page of the guide to Aliens: Fireteam Elite you will learn more about the currency in the game, including the Requisition Credits. This currency is necessary to unlock new abilities or weapons.
Every time you succeed or fail to complete a given chapter, you receive experience points in return, but also Requisitions Credits. These credits are very important, because with them you will be able to buy new weapons or various weapons add-ons, such as silencers or magazine. In addition, you can buy perks and useful items, such as sentry turrets or an anti-personnel mine.
If you want to earn a lot more credits, try to challenge yourself by choosing the challenge card that increases the credit multiplier. This will give you a lot more credits, and as a result a faster access to your desired items, modules and weapons.