Dot IO (.io) is a top level domain extension which was first introduced in the year 1997 as a country code top level domain name. It was meant to cover entities connected with British Indian ocean territory. But, it was later widely used as a reference to input/output, and startups has started using it widely. And, in the recent year .io domains are started to have a good aftermarket potential. Park.io is the marketplace where it’s traded often.
Here is an analysis of .io domains – data courtesy: Namebio