What is locking diconnect with respect to starters.

A motor starter is a "controller" which usually is a magnetic device that cannot be locked out mechanically.

A disconnect is a manual switch that will have some sort of hasp or method of adding a lock to the handle to prevent the disconnect form being turned on .

Any motor must have a disconnecting means no more than 50 feet away AND within sight of the motor, or the disconnecting means may be remote AND be lockable.