On this page of the guide to Aliens: Fireteam Elite you will learn about what grind is. Additionally, you will see how important it is in case of Aliens: Fireteam Elite.
Grind by definition means repeating certain actions. Its purpose is to gain more experience, or e.g. to get access to new content more quickly. In case of Aliens: Fireteam Elite it's no different. Grind is very important in this game.
To unlock everything in the game, you need to repeat specific missions often only to gain more and more experience points and credits, so that you will be able to get new weapons, add-ons, challenge cards, and even emoticons or skins more quickly.
Therefore, it's important to do many different since the beginning of the game to get credits or other items.. You can also make the missions harder for yourself to increase the multiplier, so that you won't have to spend so much time grinding.