Generally, in order to serve an individual with a summons and complaint, most states require the individual to be personally served. In other words, the relevant documents must be handed to the individual. However, if after a few reasonable attempts to effect personal service on the individual, the sheriff or the process server is unable to do so, the plaintiff or petitioner may ask the court to grant a motion for alternate service. If the court grants the motion, then the plaintiff or petitioner may be allowed to mail the documents to the individual, tack the papers on the door of the individual's residence, or serve the papers on another individual in the household.
In short, when a plaintiff wants to sue someone, but the plaintiff is unable to find him or her, the plaintiff can serve the papers on the defendant by one of the alternate methods listed above so long as the plaintiff gets an order for alternate service from the judge.
If you received an order for alternate service, there were most likely other papers attached to it. The next step you should take depends upon what those papers are. You should contact an attorney licensed in your state to help evaluate your options.