AT stands for Ampere- Turns.
This term is used to measure the sensitivity of a reed switch or the magnetic field needed to close the gap between the blades of the reed switch.
A low number like 1 means the switch is very sensitive.
If you have a 1AT switch and you walk to the North Pole it will close on its own due to the earth’s magnetic field which is about 2 AT.
Magnetic fields used to close reed switches are created by wire wound coils or permanent magnets.
Sensitivity / AT is primarily a function of the gap between the reed blades, the bigger the gap the higher the AT value /sensitivity.
Other factors are blade size and blade material. All reed switch manufacturers measure AT differently. It is best to sample switches with a full range of AT values in your application to determine what truly works.