User modes (mostly shortened to Umodes) are settings that can be placed on your nickname that have some effect to your session on chat - Some effect channels your in, others effect peoples ability to pvt or notice you.
Think of Umodes as the oppasite of channel modes, they are set on YOU not the channel(s).
They are mostly for protection purposes but serve other purposes as well, like providing information about your connection.
You can find a list & a brief explantion of the Mode Name & its purpose by typing:
/raw help Umodes <- mirc only command
/help umodes <- this should work for every other client/program used to connect