This is a communication framework that devices or softwares among devices use to communicate and pass information with one another. In particular, DDS is what runs ROS 2 (The backbone of ROS 2). It's what allows different Nodes (whether they are on the same computer or split across a robot and a laptop over house WiFi) to find and talk to one another.
DDS uses network communication, such as via house WiFi. The underlying protocol that it can use can be User Datagram Protocol (UDP) or Transmission Control Protocol (TCP).
For ROS, there is FastDDS and Cyclone DDS, and many other.