Elizabeth Proctor lies during her testimony. As a result, John's testimony against Abigail is discredited. The second result is that John and many others are sentenced to die, because the judges now fully believe Abigail.
The irony of the situation is that Elizabeth sought to protect her husband, but she ultimately condemned him to die. It's also ironic, because the audience expects her to tell the truth.
Proctor: In her life, sir, she have never lied. There are them that cannot sing, and them that cannot weep - my wife cannot lie.
John tells the court that Elizabeth will tell the truth no matter what. She has hurt him before by telling the truth, and he expects it to happen again. The audience expects her to tell the truth based on John's statement. And then she lies. She lies to protect John's name in the community, and her lie does the opposite of protecting John.