A fictitious defendant is a person that cannot be identified by the plaintiff before a lawsuit is commenced.
As the statute of limitations for many torts such as medical malpractice is generally very short, plaintiffs under pressure to issue an originating process such as a statement of claim often use contrived names such as John Doe in the title of proceedings and identify the person's role in the lawsuit in the body of the pleading.
Generally, this tactic preserves the limitation period and, with leave of the court, the plaintiff can later substitute the real name of the defendant once it is learned during the discovery process.
For example, in a medical malpractice case, the plaintiff may have been treated by doctors and nurses and did not know the names of those providers at the time.
At the beginning of the lawsuit, it may be impossible to determine which medical professional was negligent, so all persons who treated the plaintiff must be sued.