Bit rate is the amount of data that can be transfered at any time. Despite what the screen shows, the codec is important in limiting the bitrate. DVDs suffer severely from low bitrate, but Blu-ray has solved this problem.
This is more about data transfer and less about resolution. A codec can compress an image so the data is not as clear as it could be but will fit within a set bitrate.
http://en.wikipedia.org/wiki/Bit_rate#Video_.28MPEG2.29