The term refers to who, as a matter of law, has the right to bring a legal claim, a lawsuit, an arbitration. So as to prevent a legal ‘free-for-all,’ most legal systems limit the number of persons who can bring a claim in any given situation. Those persons are said to have legal ‘Standing’ or locus standi.
The general principle of standing is that a person bringing a lawsuit or other legal claim has to have suffered some actual injury from the conduct or act complained of, and that injury cannot be too remote in terms of causation (there cannot be too long a chain of causation) and usually has to have been an injury that was a reasonably contemplatable result of that conduct or act. In contract and arbitration law the claimant usually has to be a party to the contract or arbitration agreement, or depending on local law, an intended beneficiary. Standing rules can be very complex and can vary subtly from one legal system to another.