Collocations are words that normally go together in written and spoken English. They make your English more fluent and native like. If you use the wrong collocation, people would say – “Well, it doesn’t sound right. They don’t say it that way in English.”
The tricky part is that there are no English grammar rules stipulating how and when certain words go together, you simply have to develop “the feel” of how words are naturally used. Basically you have to learn English collocations and incorporate them into your spoken and written English.
For instance when you go back to work after a few days of illness, you’d tell your work colleagues that you’re fully recovered. If you use any other word with recovered – completely recovered, absolutely recovered, totally recovered – it doesn’t sound as good as the natural collocation fully recovered. The two words fully and recovered are the ones that naturally go together in English language, so can say that those words collocate with each other.