Idioms are a group of words that are established by usage and are unique to the language. It usually has a different meaning than what it reads. It is a set expression or a phrase, and it does not hold the same meaning when read separately. Idioms do not have literal meanings.  
A snake in the grass
In general terms, it has its usual meaning. But when used as an idiom, the meaning refers to a disloyal person who pretends to be good.
To come off with flying colours
It means to emerge out with brilliant success, but not the literal meaning of coming out with a colourful display!
Next, let us look at some more idioms with meanings:
Boil down toAmount toThe facts of the case boiled down to a mere fraudulent act
Get into hot waterGet into difficulty
James got into hot water because of his laziness
Shed crocodile tearsPretend to cryHe shed crocodile tears when his boss  scolded him
Peas in a podStay together alwaysSheela and her sisters are like peas in a pod
Green with envyExtremely jealousHe was green with envy when his neighbour secured the first rank
Idioms convey subtle but coherent meaning for the sentences. For instance, "shed crocodile tears"- how is it coherent? Since a crocodile cannot cry, it cannot have any tears. So it means false tears or pretending to cry.