Split Horizon is the term used to define the process of leaf nodes in a fabric dropping duplicate traffic received from other leaf nodes in the same ES. This prevents traffic looping and duplication of traffic back to the originating host.
When a multi-homed host transmits BUM traffic to a leaf node, the leaf forwards this traffic to other leaf nodes in the same ES. The other nodes in the same ES drop this received traffic to prevent looping and packet duplication to the originating host. Works along with Local Bias.