Replica is an item property denoting a copy of the original item. Replicas have less durability than the original piece and they cannot be improved by Powder of Fortification.
Replicas are one of the possible Champion Spawn Artifact reward items for completing Champion Spawns. They are copies of items originally handed out at EM Events with the exception of the additional "replica" tag in the description and first appeared with Publish 57.
They were introduced as rewards for doing Champion Spawns outside of the Felucca facet and as an effort to combat duping of the original items, which were previously very rare and difficult to get since the original EM program concluded several years ago.
Additionally, the original non-replica versions of these items are still considered rare.