In short: E-UTRAN New Radio - Dual Connectivity.
It’s defined by 3GPP. (3GPP and ITU Tutorial)
The scenario is when you have an LTE eNodeB acting as a master node, and a 5G gNodeB as a secondary node and you want to allow UE (User Equipment) to access both LTE and 5G simultaneously on the same spectrum band.
Of course, the UE needs to be ENDC capable (support ENDC).
The main reason/advantages is to have less ping-ponging for the UEs (they are already “connected” to both LTE and 5G).