An anchor ASIN is an ASIN already listed under the parent that is also in your inventory. This ASIN is a child listing that downloads in the template with the parent and orphan, and when you upload the template it is what tells seller central what parent to join the orphan to.

Row 4 is the parent listing, row 5 is the anchor- when listings download through the variation wizard they have the UPC instead of the ASIN, and row 6 is the orphans in this example.

If you download a template and it only has the parent and the orphan, when you upload it, it will create a new parent instead of joining the orphan to the intended parent.