Dynamic Spectrum Sharing (DSS) is a technology that allows spectrum to be used for both 4G and 5G connections simultaneously.
Conventionally, network operators deploy up-and-coming cellular technologies on spectrum that they’ve cleared of connections based on older technologies. DSS will allow network operators to skip the clearing step and use spectrum more efficiently. Verizon is making extensive use of DSS to offer its low-band 5G.
The video below from Rohde Schwarz offers a more technical introduction to DSS: