Discovery is evidence that is required to be disclosed to an opposing party. Supplemental discovery is when there is additional evidence that has been provided to the other side, after the initial discovery. Sometimes supplemental discovery is another witness statement, an additional report, photos, recordings, and such. In all likelihood, one the parties (usually the State) provided supplemental discovery to the opposing side and provided a notice to the court to reflect that the additional discovery has been provided.
This is not to be considered legal advice nor does an attorney-client relationship exist.
It means that there is additional information that was given to your attorney in your case. It can be new witnesses, new physical evidence, an oversight of information, etc. Contact your attorney to determine what the evidence was and if you're pro se and didn't receive anything from the State Attorney, you may want to contact the Clerk and the State to change your address.
The state attorney has a continuing obligation to provide the Defendant notice of what evidence the state attorney has in its possession. The state attorney has this obligation up until the jury has rendered a verdict at trial. Theoretically, "supplemental discovery" is evidence that the state attorney only recently came into possession of.
What likely happened is that a prosecutor at the State Attorney's office finally looked at your case file and realized that they needed to get on the ball with your case. They though to themselves, "oh crap, I'm not going to be able to prove the charge without this and now I need to disclose it to the defendant's attorney." Definitely consult with your attorney to understand what impact this may have on his/her trial preparations. If the supplemental discovery is late and prejudices your case, he should try to exclude the evidence if the judge allows it. The entire process could also impact the plea negotiations so you should be concerned enough to discuss it with your defense attorney. In my experience, this usually means one of two things: (1) the prosecutor is way behind and unorganized so you may want to tee the case up for trial, or (2) they dropped the ball originally but now have their case and witnesses and evidence together and they're eager to proceed to trial. Good luck.