Idioms are fixed expressions whose meanings cannot be understood from the meanings of the individual words. For example, 'raining cats and dogs' means raining very heavily, 'break the ice' means to start a conversation or social interaction, and 'cost an arm and a leg' means to be very expensive. Native speakers learn idioms through exposure, and they are essential for natural communication and understanding native speech.
When you learned vocabulary, you learned that word meanings combine — "red car" means what "red" means plus what "car" means. Idioms are the exceptions that prove the rule. An idiom is an expression whose meaning is non-compositional: the whole does not equal the sum of its parts. "Kick the bucket" has nothing to do with feet or buckets — it means to die. "Break the ice" has nothing to do with frozen water — it means to initiate interaction in an awkward social situation. The individual words are transparent; the idiom's meaning is opaque until you know the convention.
This non-compositionality is also why idioms are lexically fixed. You can say "it's raining cats and dogs" but not "it's raining dogs and cats" or "it's drizzling cats and dogs" — the expression resists modification that would be perfectly normal for a literal phrase. Similarly, "spill the beans" (reveal a secret) can't become "pour the beans" or "spill the lentils." Idioms behave more like single vocabulary items than like assembled phrases: they are stored as chunks, not generated fresh each time.
The practical consequence is that idioms must be learned through exposure rather than analysis. No amount of knowing what "bite," "the," and "bullet" mean individually will tell you that "bite the bullet" means to endure something painful or unpleasant with fortitude. This is why vocabulary building is a prerequisite — not because you decode idioms from their words, but because a broad vocabulary gives you context to encounter idioms frequently enough to absorb their meanings.
Idioms vary significantly by register, dialect, and era. "The whole nine yards," "kick the can down the road," "par for the course," and "hit the ground running" are common in professional English but may be opaque across dialects or to non-native speakers. Idiomatic competence — knowing not just what an idiom means but when and with whom to use it — is one of the markers of advanced language proficiency. Over-use, misuse, or misapplied idioms ("I heard it through the grapevine, so take that with a large grain of salt") can undercut clarity. Fluency with idioms signals not just vocabulary depth but cultural immersion in how a language community actually speaks.