A more thorough explanation:

A turncoat witness is someone who is called to testify in a trial and is expected to provide helpful information for one side, but then changes their story and becomes unhelpful or even hostile to that side. This can be a problem for the litigating party because they were counting on the witness to support their case.

For example, in a court case called People v. Brown, a witness initially told many people that the defendant had beaten and kicked her. However, when she was called to testify in court, she changed her story and said that she had actually been the aggressor and had injured herself while attacking the defendant. This kind of witness can be very frustrating for the party that called them, because they were expecting the witness to help their case, but instead the witness ended up hurting it.

When a turncoat witness appears in court, the other side can use the witness's previous statements to try to show that they are not telling the truth. This is allowed under the rules of evidence, which are the rules that govern what kinds of information can be presented in court. For example, in California, where the Brown case took place, there is a rule that allows a party to use a witness's previous inconsistent statements to try to show that they are not telling the truth.

Another example of a turncoat witness might be a witness who initially agrees to testify for one side, but then changes their mind and decides to testify for the other side instead. This can be a problem for the first side, because they were counting on that witness to help their case, but now they are working against them.