This is a question that's often asked of defense attorneys when someone is sitting in jail and they are scheduled for a Preliminary/Felony Hearing. When you are arrested for a felony, you have a right to a preliminary/felony hearing to determine if there's a good reason to hold you for the action of the Grand Jury (for presentment of your case to the Grand Jury to see if they are going to Indict you for a felony or not).
Before that happens, the court gives you the right to a preliminary hearing to determine if there's reason to hold you on bail so you don't flee while the Grand Jury deliberates. Such a hearing is often not held because if you force the hearing to be held, the prosecutor will then intensify his/her plea negotiations if you insist upon such a hearing, so the defense attorney will often waive the hearing in order to keep the prospect of plea bargaining alive if needed later. Also, if you're out on bail, its not as much of a necessity so they waive it.
So when your attorney comes to you to sign away your right to a felony hearing, its not the end of the world to sign it - unless the charges are completely bogus and there's no way they can justify holding you in jail any longer. Nonetheless, be advised that most jurisdictions allow the attorney to sign the document anyway - without the Defendant's signature - if they wish. So, often its the best thing to simply sign it and let your attorney handle the matter in the proper fashion he/she determines.