What does the quote Time shall unfold what plighted cunning hides Who covers faults at last shame them derides mean?

Cordelia bids her evil sisters farewell, telling them she knows what they really are like: "[I] am most loath to call/ Your faults as they are named [271]." Cordelia gets the last Word on the exchange with her sisters at line 282: "Time shall unfold what plighted [covered] cunning hides, /Who covers faults, at last shame them derides." Those who hide their evils will have them finally exposed and suffer shame. A foreshadowing of what is to come!

*This explanation is from an English lecture published by Signet Classics.

That is not exactly correct. Cordelia is not necessarily addressing her sisters with this line. She could as easily be addressing the audience, telling them what to expect, then she turns back to her sisters for the last line, "Well may you prosper!" This choice is better because she is speaking aphoristically in the first two lines. Time UNFOLDS what PLIGHTED cunning hides: meaning, that eventually the passage of time will expose (unfold) things that people deceitfully, deviously keep hidden in the folds (plighted) of time, like dirty secrets. But also note that "time" is the subject and the action is time's, so it is better to stick with the first folio and say "at last with shame derides," meaning in the end time will ridicule those people with shame, that is, when the secrets are exposed. The power of Cordelia's adage also sets her up as a kind of soothsayer, foretelling the outcome of the play. In this case, there is also a second meaning...which if you understand the story correctly you can figure out...