OSC is a protocol for communicating between different pieces of hardware or software over a computer network. QLab is created the ability to add or edit cues utilizing OSC.
You can have QLab send an OSC command to itself or receive one from someone else. The quick and easy way to do this is to send Slash New Space and the cue type in Quotes
So this OSC cue, when run, will create a new Start cue.
Why would you ever need to do this? Well, for example, this could allow you to create your own custom interface on your iPad utilizing an app such as TouchOSC.