An ASIN (Amazon Standard Identification Number) is the number Amazon assigns to a product in their catalog. Every product page on Amazon automatically gets an ASIN when that product page is created.
An offer is the posting you will make under a product page. The ASIN lets Amazon know where to place your item.
When you have an item you want to post to Amazon, you must first find out if the product already exists on Amazon. If it does exist, you will want to post your item under that ASIN
Here's a longer explanation about why you want to post under existing ASIN's:
If it does not exist, you will need to create the product page and Amazon will assign an ASIN to it.
Here's our current guide for how to create an ASIN: