What does the meaning of the term "Ampere Turns" and "Ampere-turns per metre"

Ampere turns is the unit of Magneto Motive Force of a magnetic circuit, the equivalent of emf in electrical circuit. The MMF is measured as the product of the dc current flowing through the circuit and the number of turns. The higher the number of turns in a coil, higher will be the magnetomotive force for the same current.

However, this relationship holds true only till the core of the coil gets saturated, after which there is no change in mmf for an increase in current.

The field strength of a coil is the magnetomotive force per unit length. This is measured as ampere-turns per metre.