Privilege against self-incrimination is a freedom granted to witnesses in court cases by the Fifth Amendment, which prevents them from having to give testimonies that could incriminate themselves.
In other words, privilege against self-incrimination is the right of citizens to not be forced to give evidence that could cause their own arrest and prosecution.
This privilege is often very important in trials because it can determine whether or not a witness will give certain evidence during a testimony. For example, if a witness knows crucial information about the defendant, but providing this evidence would incriminate themselves, then they would not have to give it.
This privilege is also designed to help preserve the accuracy of testimonies. The reason is because if witnesses were forced to give self-incriminating testimonies, they could very easily be tempted to lie instead.