Trade secrets are a method of protecting an idea by only keeping the method or device privately held to you and close colleagues. Very few things can be protected this way, due to the ability of people to reverse-engineer and develop their own versions of a similar concept with relative ease. Flavors and methods of manufacture are often protected this way. It is extremely difficult to reverse-engineer a flavor, so the ratios of the ingredients in Coca Cola and Pepsi, for example, are protected using trade secrets.
Another example, Apple Computers is notorious for not giving tours of their manufacturing facilities. Rumors are that they have developed highly efficient manufacturing processes for their devices. Apple, keeping their manufacturing processes a trade secret, ensures that if someone made a competing product (Samsung, etc), they likely would not have as efficient manufacturing as Apple, and would be at a cost-basis deficit when competing in the marketplace.
The problem with Trade Secrets is if someone does happen to figure out your trade secret, you have absolutely no legal protection against them to use it for their own benefit. If someone figures out the recipe for Coca Cola, they could effectively enter the marketplace with the same product, and Coca Cola would have no recourse.
However, if your invention is something difficult to reverse-engineer, you have protection as long as you can keep a secret, or as long as no one else has figured out what you have figured out. Coca Cola has kept their various cola recipes secret since 1886!