First of all you need to understand what it means by turnkey. Quoted from About.com,
A turnkey business is a business that includes everything you need to immediately start running the business.
InvestorWords.com defines a turnkey operation as “A product or service which can be implemented or utilized with no additional work required by the buyer (just by ‘turning the key’)”.
A business that is being sold as a turnkey business would include tangibles such as inventory and equipment through intangibles such as a previously established reputation and goodwill.

So if we are talking about turnkey website, it would mean that the website is ready to be used as promotion and marketing tools for your business.
For more understanding, read this article from BasicTips.com on what is turnkey website.
“A turnkey website is a completely functional, operating site with hosting and pre-installed interactive scripts for forms, shopping carts (with product images, descriptions, prices), affiliate programs, etc.”
So if you are planning to start a business and would like to have website but not too inclined to learn the technical aspect of a website, you should consider buying a turnkey website.
(Photo by mmagallan)