Dual connectivity means mobile terminal will be connected to 4G as well 5G NR base station.
The UE attaches to the LTE network first. Then the UE signals to the network that it can simultaneously connect to the 4G and 5G networks. The 4G eNB and 5G gNB exchange message to set up the bearer on the 5G gNB. The UE will then have bearer set up for 4g as well as 5G NR.