Does it mean that it simply must be able to become patent, and not have a similar patent already filed?
It means the idea has to be new (never published) and inventive (not obvious from the state of the art).
I would expand this further, but there are plenty of websites, articles, books, .. explainign this better than I could. Please look up (above mentioned) requirements for patentability. You could google them or click on the patentability tag and start there.
In addition to DonQuiKong's answer, the desire for patentability is motivated by the fact that a patent provides a period of exclusivity which may be enormously valuable.
If someone is going to invest significant capital developing a product they want to ensure sufficient payback. A patent provides typically 20 years of protection from the application date.