As the others mentioned, beefcake was talking about muscle failure.
It means that by the last rep your muscle(s) should be so fatigued that you can barely complete it properly, if at all.
So if you are doing bicep curls, by your last rep you should
really be feeling it and doubting you can even lift that weight one last time.
If you don't feel that, then the weight you are lifting is too light and you should move up to a heavier one next time.
