The aspect ratio (AR) of a jump ring is found by dividing the inner diameter (ID) of the ring by the size of the wire used to make the ring. This ring is 19g AWG (1mm) inner diameter (ID) 3.25, so 3.25 ÷ 1 = 3.25. This ring has an AR of 3.2.
Knowing the aspect ratio (AR) of a ring allows you to substitute different size rings into a pattern. Changing the rings used in a pattern to a different AR will change the look of the weave. A bigger AR will make the weave looser, a smaller AR will make the weave tighter. The same AR will give a similar look and feel to the weave, but the size of the finished product will change. So you can scale the size of a pattern up or down. For some complex patterns having the right AR is crucial for you to be able to make the chain maille weave.
The ID of a ring can be either the measurement of the mandrel it was made on, this is the measurement you will find when you are buying rings, or it can be the actual inner diameter measured with callipers. The measurement is different because metal springs back slightly after being wound around the mandrel. For this ring example the actual ID is 3.5mm instead of the 3.25mm mandrel measurement. So the actual AR is 3.5 ÷ 1 = 3.5. The difference isn't massive. Either one will give you a good indication of this number.