Generally, when you are "pre-approved" for a credit card, it simply means that a preliminary review of your credit record by the card company indicates you may meet the credit union’s criteria. You are not absolutely guaranteed to receive a card -- or the credit limit, interest rate, or other terms featured in the promotion.
You will only receive the card if you apply and you meet the credit card company’s established criteria. These other criteria -- including making sure that your credit record has not deteriorated since you were selected for the offer -- will determine whether you get the card, and if you do, the credit limit, interest rate, and other terms for the card.
You should read the promotion very carefully before applying.