The 160 is the width of the tire. 160 mm. Of course, there is some variation from brand to brand and model to model. Also, depending upon the width of the rim it's mounted on, the tire width will then vary just a bit also. A narrower rim will make the tire just a bit narrower than the same tire mounted on a wider rim.
To find out what rim widths a certain tire is recommended for, just go to the tire manufacturer's web site and check the specs for that particular tire and tire size. Almost always it's listed. Easy information to find out.
If you want to actually measure a particular tire, get yourself some large calipers or something and make a visit to your local motorcycle shop. But remember, mounted they may be a bit different. I recommend measuring the maximum width of the tread and then also measuring the width of the bead of the unmounted tire. That may give you an idea of how the max width may change when the tire is mounted.