When a host device is connected to multiple leaf nodes through a set of ethernet links, then this set of links is considered as an Ethernet Segment (ES). Each ES is identified by an unique non-zero 10-octet hexadecimal identifier. This identifier is called the Ethernet Segment Identifier (ESI). This ESI value is used in the ES EVPN route to discover other leaf nodes belonging to the same ethernet segment.
Single homed devices do not require to have an ESI value assigned to them.