Core is a term for the main memory directly referenced by the CPU. It was named because originally computer main storage consisted of magnetic toroids weaved into a matrix with write and sense wires. To write a bit the coresponding toroids x and y write wires would be energized with a particular polarity current. To read a toroid the write wires would be energized to set the toriod to a zero and if it had been previously magnetized to a one a small current would be induced in the sense wire. If it were already zero magnitized no sense current would be produced. This means that if the toroid had been set to a one reading it would set it to zero and the circuitry would have to re-write a one to re-establish it's correct value.