Idioms are natural manner of speaking to a native speaker of a language.
According to the Cambridge Dictionary Online, an idiom is a group of words in a fixed order that have a particular meaning that is different from the meanings of each word on its own: To “have bitten off more than you can chew” is an idiom that meansyou have tried to do something which is too difficult for you.
Since the meaning cannot be deduced by its words, it is interesting to have at hand an Idiom Dictionary: http://www.idiomsite.com/
Finally, I would like to highlike an idiom which is similar in Spanish:
A Bird In The Hand Is Worth Two In The Bush:
Having something that is certain is much better than taking a risk for more, because chances are you might lose everything.