A surrebuttal is a response to a rebuttal. Typically, in a trial, a plaintiff presents their case. Following this presentation, the defense presents its case. Then the plaintiff can respond to the defense's argument with a rebuttal. Finally, a surrebuttal can be given by the defendant.
However, if the plaintiff would then like to respond to the defendant's surrebuttal, the judge will often allow the plaintiff to make a surrebuttal as well. Technically, then, a surrebuttal can be made by either a plaintiff or a defendant, and it can be a response to a rebuttal or to another surrebuttal.
Surrebuttals exist so that each party in a lawsuit can have an opportunity to respond to points made by the other in its rebuttal or surrebuttal. Trials allow surrebuttals so that each party has a chance to express its argument and to address the points made by the opposing counsel. This enables all relevant points to be covered and responded to, thus allowing the judge or jury to have all of the information that they need to make their decisions about the verdict.
An example of a surrebuttal would be the defense in a murder case responding to new evidence that the prosecution provided in its rebuttal.