What does archiving a card do?

Archiving is essentially telling the system that you are already familiar with the material on the card and will not be forgetting it. You are telling us that you no longer want to see that specific card in your reviews.

By archiving cards, you will reduce the number of reviews contributing to your review threshold, which allows you to move through material you already know more quickly.

In case you've accidentally archived a card you didn't want to, you can find them by going to Library > Flashcards. From here, select a deck and change the Status dropdown to Archived. You can also enter other search parameters to filter the results down further. Once you've found the card, you can manually unarchive it by selecting the checkbox to the left of the card, then clicking on Unarchive Selected at the top of the list.