The following images will help break down how I J arcs movements work. 

The I movement is the X offset from the centre point of the arc

The J movement is the Y offset from the centre point of the arc

"Relative Arcs" are X and Y movements based from the start point of the arc. This is the most common method. 

"Absolute Arcs" are X and Y movements based from the 0,0 origin.

Example using Relative Arcs

Example using Absolute Arcs