A collocation is a group of words that usually go together. For example, in English, we usually say ‘heavy rain’. It’s grammatically correct to say ‘strong rain’ or ‘big rain’, but both of these sound completely strange. A native English speaker would never say ‘big rain’.
“This is strong coffee”.
The collocation here is strong and coffee. But what happens if we replace strong with a synonym like powerful? Can we say powerful coffee?
To a native speaker powerful coffee sounds strange and incorrect. So we need to know which words will work together to make word combinations that sound correct.