The ISO rating stems back to the film era and knowing its origins will help understand how it works with DSLRs.
What does ISO mean?
There’s some history involved here, so bear with this explanation!

There's a bewildering range of cameras on the market, but if you follow some simple guidelines you can narrow down the choice. The first thing is that if you're considering spending a couple of hundred dollars or more on a camera, we'd always recommend buying from a physical shop rather than the internet.

The choice of ISO rating you use has an enormous impact on the final result of your images. Its main influence is that it will affect what combinations of apertures and shutter speeds are available to you, for instance, in low light.

While every camera brand designs digital SLRs in their own way, there are many similarities in the control layouts of most models. We’ve highlighted the key functions that you should get to know as soon as possible, as understarding how your DSLR works will help you to capture great pictures with ease.