A DUI preliminary hearing is the "trial before the trial". During this "mini trial" the judge will determine whether there is enough evidence to require the defendant to stand trial. During this process the judge utilizes the "probable cause" legal standard. This standard requires the judge to ask himself whether there is enough evidence to convince a reasonable jury that the defendant committed the crime he is charged with.
During the preliminary hearing, the judge listens to arguments from the prosecution and the defendant. Witnesses may be called and physical evidence may be introduced by the prosecution during this part of the DUI process. The defense attorney can typically cross-examine the prosecution's witnesses and question the accuracy of their evidence. All this is done with an eye towards helping the judge decide whether or not there is probable cause.
Most DUI cases do not reach the preliminary hearing stage because most defendants plead guilty upon arraignment because usually the evidence of DUI is very strong. Furthermore, many states do not permit preliminary hearings for DUI's unless a felony is charged. Other states utilize a "grand jury indictment" process in which a designated group of citizens decides whether, based on the prosecutions evidence, the case should proceed to trial.
If you are in a state that permits a preliminary hearing for DUI cases, you should hire a DUI/DWI lawyer to help you with the preliminary hearing. A good DUI attorney will know the right questions to ask at this stage of the DUI process, may be able to put some holes in the prosecution's case, and may be able to lessen the severity of the charges.