matching numbers means a couple things in the Mustang world.
If your engine and tranny were stamped with your cars vin number at the factory then that is matching numbers. Most early Mustangs engines and tranny's were not vin stamped.
The more "proper" why to describe an early Mustang as having matching numbers is show that the parts are proper date coded for the scheduled assembly date of the car. For example, the motor has a casting date within a couple of months before your car assembly date. Or your fenders or hood, etc were stamped within a couple of months before your car was assembled.
1965 Mustang 2+2
1989 Mustang GT Convertible
MCA certified judge for 65's and late models