You are a beginning English learner. You enjoy the methodical approach, so you tackle the language systematically, memorizing lists of irregular verbs, spelling norms, and syntactic rules. No conversational practice, no watching movies. You want to get the theory right first.
One day, you think you have mastered it. You are a walking grammar book.
Then you realize you have been so engrossed in your studies that you skipped lunch, so you ask a passer-by:
Excuse me, sir. I am heavily hungry. Could you point me to the nearest swift-food restaurant?
Which he greets with a baffled stare.
You studied the standard rules of English, but there is a part of the language (of any language) that will never fit in that tidy set of axioms
According to the Oxford English Dictionary, collocations are pairs or groups of words that are habitually juxtaposed, such as strong coffee or heavy drinker. As such, they are the final touch foreign learners (or say, machine translation systems) need to “speak properly.” You can communicate without knowing them, but you will sound pretty weird to the native ear.
In a wider sense, collocations are a sort of lightweight idioms, but they differ from idioms in a couple of ways:
Their meaning is transparent — you can guess it the first time you see them (which you can’t with proper, metaphorical idioms, such as kick the bucket or a piece of cake)