The firing end of a spark plug threads into the cylinder head. A spark plugs threads are comprised of 3 dimensions: Reach, Diameter and Pitch.
Thread pitch is the distance between threads expressed in millimetres. For example, a thread pitch of 1.25 means the distance between each thread is 1.25mm.
When a thread size is expressed in a set of two numbers, for example, 14x1.25 or M14x1.25 the first number is the thread diameter and the second number is the thread pitch.