That depends on how the plaintiff filed the case. If it was filed as a small claims case in IL then the small claims summons form tells you in part 4 what you need to do: either you would need to show up on the return date shown on the summons or file a written appearance. The form will tell you the time and date that you either need to appear at the court or the time and date your written appearance must be filed. If you fail to show up or file the written appearance as directed you risk a default judgment against you. From what you've said it sounds like you've received a small claims court summons and have been directed to appear in court for the return date. Illinois Legal Aid has a page with information about the basics of small claims court cases. While written more for the plaintiff, the information in it is useful for defendants too. Also look at Illinois Supreme Court rules 281-289, which are the rules that apply to small claims court cases.
If this is not a small claims court case then the procedures will be a bit different. So if you aren't in small claims court you'd need to tell us what court the case is filed and someone here may be able to give you specific information on that.

