Inductance is what opposes change in current in a circuit.
In electronics the letter L represents inductance. Inductors are devices which oppose any change in current regardless in what direction. If you put a core inside, that's something that the coil is wound around all the time, like a pencil or a nail, that is exactly what a core is. So, as the current flows through the circuit, a magnetic field will induce in the coil. This field will start from 0 and then will expand to it's fullest value or flux density.