What are the registration requirements for .IO domains?
There are no restrictions in buying .IO domains.
What does .IO stand for?
.IO represents the official domain extension or the internet country code top level domain (ccTLD) of British Indian Ocean Territory. NIC.IO is the authorized body for carrying out its registry and administration. The domain is currently being treated by Google as a generic top level domain since webmasters and internet users don’t consider it as a country code domain.

Aside from Indian Ocean, IO is also used as an acronym for other words. Other popular meanings of IO include: input/output (computing), interest only (accounting), information online (internet), information operation (military), instant offense (police), India Office (government) and investor optimum (stocks).
Why register a .IO domain?
British Indian Ocean Territory is a territory of the United Kingdom located between Indonesia and Tanzania. Commercial fishing is the primary source of revenues amounting to one million US dollars per annum. Agricultural or industrial activities do not exist. Construction projects for defense facilities are carried out by contractual workers from the Philippines and Mauritius. Currently, there are less than 5000 civilians and military personnel living in the islands.

Although BIOT is not a tourist destination and is also not a place for making investments, there are plenty of reasons why .IO is a profitable domain suffix for your website.
It is ideal for companies wanting to register a domain name that is already taken in .COM or is being sold at a hefty price. The keyword that you want to use for your website name is likely still available in .IO.
Although IO is a ccTLD, Google does not treat it like one because majority of websites using this country code are not relevant to Indian Ocean. If your company is not exactly targeting consumers in IO (the population there is very low), IO domain is a good substitute for generic TLDS like .COM, .NET or .ORG.
IO is good for domain hacking. There are other popular meanings of IO aside from Indian Ocean. If your business is engaged in any of those possible IO definitions, you may take advantage of that.
Using IO for global targeting is ideal. In contrast to other ccTLDs which are useful for local targeting, IO domain suffix does not have any effect on local search results. Using this country code does not mean that most of your traffic will come from British Indian Ocean. This is good if your goal is to generate traffic from all over the world and not just from a specific location.
IO is ideal for companies engaged in technology, software or computing. For some reason, webmasters and internet users associate IO to IT (information technology) probably because most websites using this domain extension are related to file or media converter.