Personal jurisdiction refers to the power that a court has to make a decision regarding the party being sued in a case. Before a court can exercise power over a party, the U.S. Constitution requires that the party has certain minimum contacts with the forum in which the court sits. International Shoe v Washington, 326 US 310 (1945). So if the plaintiff sues a defendant, that defendant can object to the suit by arguing that the court does not have personal jurisdiction over the defendant.
Waiving Personal Jurisdiction
Personal jurisdiction can generally be waived (contrast this with Subject Matter Jurisdiction, which cannot be waived), so if the party being sued appears in a court without objecting to the court's lack of personal jurisdiction over it, then the court will assume that the defendant is waiving any challenge to personal jurisdiction. See Federal Rule of Civil Procedure 12(b)(2).
Obtaining Personal Jurisdiction
Typically for a court to have personal jurisdiction over a defendant, the plaintiff needs to serve the defendant in the state in which the court sits, and the defendant needs to voluntarily appear in court.
Federal Rules of Civil Procedure Rule 4(k) describes whether a state's courts would have the authority to adjudicate a claim as it relates to personal jurisdiction.
See also in personam