Stands for "User Datagram Protocol."
UDP is a data transport protocol within the Internet Protocol (IP) suite. UDP is a connectionless protocol that does not rely on two hosts establishing a connection before transferring datagrams. Streaming media, online gaming, and other situations where low latency is more important than the occasional dropped data will use UDP.
Compared to other data transfer protocols like TCP, SSH, and HTTPS, UDP is a simple way to send data. The device sending the datagrams does not need to establish a connection, ask whether each packet has been received, and re-send any data lost in transit. Instead, it sends a series of datagrams containing the destination IP address, routing information, and payload as fast as possible. Skipping the connection and verification steps means that UDP has less overhead and latency than other protocols.
However, there is no guarantee that the device receiving the datagrams is getting all of them. If network congestion causes some datagrams to get dropped, there is no way for the receiving device to ask for the data again. In the case of a video call over the Internet, a lower latency means that there is less of a delay between one end of the call saying something and the other end hearing it. If the connection between the two ends slows down and some datagrams are lost, the video may cut out for a moment before resuming.