I believe reliability means 'rate of success', i.e. it works reliably 0.8 or 80% of the time. So 1 is all the time, and 1-0.8=0.2 would be unreliability, which is related to the failure rate.
Look on the page for MIL-HDBK-217F
Reliability Prediction of Electronic Equipment- Notice F
One can download the pdf file.
Weibull.com is a good resource for statistics and statistical analysis.