Up - the device has been answered. When in the up state, media can flow bidirectionally between Asterisk and the device,
In NewChannel, it means that it was already in an answered state when the channel data structure was created.
It is much more likely that the actually agent answer will be on an existing channel and therefore on a NewState event. Also note that, if you already answered, for example to run an IVR, there will be no NewState for the caller, on agent answer, although there should be one for the agent and there should also be one corresponding to the bridging of the channels (I forget what this is called).
The full list of states will be in include/asterisk/channel.h.