Glossary
Ice barrier
A self-adhering waterproof membrane at the eaves, required by code in areas with a history of ice damming, that seals around fasteners and blocks water backing up under the shingles.
In practice
IRC R905.1.2 requires it to reach at least 24 inches inside the exterior wall line. On a house with deep overhangs that is considerably more than 24 inches of membrane. Whether your address needs one is a local determination.