In most felony cases the preliminary hearing is held within two or three weeks of the arraignment. At the preliminary hearing the District Attorney must put on evidence before the judge (usually by calling a witness) to show that there is a strong enough case against the defendant to go to trial by jury.
The burden of proof at the preliminary hearing is very low. The District Attorney need not prove the defendants guilty beyond a reasonable doubt, but must only show "probable cause" to believe the defendant is guilty.
The role of an attorney at the Preliminary Hearing:
A Preliminary Hearing is like a miniature trial. At the Preliminary Hearing a defense attorney may make objections to admission of illegal evidence, cross examine prosecution witnesses, and occasionally (though usually not recommended) call a witness for the defense. Cross examination at the Preliminary Hearing is vital, because answers given can be brought up later at trial if a witness tries to change their story.