The amount of data that is allowed to be transferred between your account and the rest of the internet for a period of time. Each byte can represent a character. In 10 GB there are 1024*1024*1024*10, or 10737418240 bytes.
Is this enough for your website?
10GB per month will be enough for 95% of all existing pages in the Internet.
THE TRUTH ABOUT UNLIMITED TRAFFIC.
When you buy web hosting, you are placed on a server with several other people. Each of these servers has LIMITED bandwidth, and overtraffic is penalized. Therefore, it is technically not possible to have unlimited bandwidth.
When hosting companies offer you unlimited traffic, they rely on the fact that most web pages use only a small amount of their traffic quota.
In this way, even if your site creates more traffic, the surplus will be compensated by the low traffic usage of the other sites on your server.
If you review the terms of use of such hosting providers, you will find out that there are actually limitations – for example you will not be allowed to host music/video, as downloads of music/video create much traffic.
Even worse, in some cases companies offering unlimited traffic apply traffic shaping – this means that they limit the download/upload speed of your site. In this way if several people try to access your site simultaneously, they will have to share this limited speed. This will result in their connection to the site being slow, which is highly undesirable!
Remember – all resources ARE LIMITED! Offering anything unlimited is just a marketing trick!