It's kind of a rough scale of the condition of the interface.
Reliability 255/255= 100% up and reliable
128/255 = 50% up and not-so reliable
txload 1/255 = 0-4% of traffic is coming from transmitted info. 128/255 would mean 50%of traffic is coming from transmitted info.
rxload is the same as tx except it's received data.
These values are kind of a high level dashboard on what the intreface is doing in regards to traffic. EIGRP Metrics are designed to use the reliability information, but is rarely imlemented.
FROM CISCO
Reliability of the interface as a fraction of 255 (255/255 is 100 percent reliability), calculated as an exponential average over 5 minutes.
txload/rxload=Load on the interface as a fraction of 255 (255/255 is completely saturated), calculated as an exponential average over 5 minutes.
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_command_reference_chapter09186a00801a7edb.html#wp1229983
