Can someone explain this CEL code for me.
 

P0146 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 3)

My dad is getting this code on his Jimmy. It is a general code for all OBDII vehicles.
What does it mean, or what is causing it to happen? (besides the O2 sensor which is obvious). Am I making it to hard and it just means that it is not working like it is supposed to.

I searched the web and here but all I get is every web page that has the ODBII codes listed on them. No explinations as to what causes the code to report.