Collocation refers to a group of two or more words that usually go together. A good way to think of collocation is to look at the word collocation. Co - meaning together - location - meaning place. Collocation are words that are located together. A good answer to "What is collocation?" is: Collocation is a group of two or more words that like to hang out together. Here are some examples of common collocations that you might know:
make tea - I made a cup of tea for lunch.
do homework - I did all of my homework yesterday.
Make and Do - A Good Starting Place for Collocation
I begin with make and do because they provide perfect examples of why collocation is so important. Generally, make refers to things that are created and do refers to tasks. However, there are plenty of exceptions to this rule - especially with the verb 'make'.
I made the bed. - Did I construct a bed? NO!
She made an exception. - An exception is allowance to breaking a rule, not a thing.
Make and Do are perfect examples of verbs that go together with specific nouns. A verb + noun combination that always go together are considered collocations.
More information and examples on make and do collocations
Why Do Words Collocate?
There is often no reason for a collocation. People just put certain words together more often than they put other words together. In fact, the use of collocations has become popular in English and language teaching because of corpus linguistics. Corpus linguistics study huge volumes of data of spoken and written English to come up with statistics on how often people use certain words and word combinations. Through this study, corpus linguistics has been able to define what are strong and weak collocations.
Strong collocations refer to words that almost always go together. It's possible that people might understand you if you don't use a strong collocation. However, if you do not use a strong collocation it will sound funny to native speakers. Let's return to our example of 'make' and 'do'. If you say:
I did a cup of coffee.
native speakers will understand that you mean:
I made a cup of coffee.
Correct use of strong collocations shows an excellent command of the English language, and can certainly help impress native speakers' of your ability to speak English well. Of course, if you are speaking to other non-native speakers the ability to use collocations correctly all the time becomes less important. That doesn't mean that correct collocation use is not important, it's just not AS important as something like correct tense. Image for a moment that you are speaking about a future meeting:
Our meeting was on Friday at four o'clock.
I've done an appointment at four o'clock for the meeting room on Friday.
In both of these sentences, there are mistakes. However, in the first sentence instead of using a future tense, the past tense is used. If you want your colleagues to come to the meeting, this mistake is very serious and will lead to no one coming to the meeting.
In the second sentence 'do an appointment' is a misuse of a strong collocation. However, the meaning is clear: You have scheduled a room at four o'clock. In this case, a mistake in collocations is not nearly as important as a mistake in tense usage.
Here are examples of strong collocations that you might not be familiar with:
high earnings (not big earnings)
long-range planning (not long-time planning)
urban guerrilla (not city guerrilla)
Why are Collocations Important?
There is an entire world of collocations to explore. Learning collocations is important because you begin to learn words in larger groups or 'chunks' of language. Putting together these chunks of language leads to more fluent English.
More information on other word groups in English