Collocation refers to how words naturally go together or form fixed relationships.
Correct: heavy rain
Incorrect: thick rain
Correct: high temperature
Incorrect: tall temperature
Correct: scenic view
Incorrect: scenic picture
Correct: have an experience
Incorrect: do/make an experience
Correct: She has blonde hair.
Incorrect: She has beige hair.
Correct: She was discharged from the hospital.
Incorrect: She was released from the hospital.
Strong collocations are where the connection between the words is fixed and restricted.
whisk an egg
Weak collocations are where a word can collocate with many other words.
big/enormous/large + house/lorry/cup
fast/shiny/expensive + car/motorbike/aeroplane
very/really/extremely + interesting/hot/generous
brown/straight/long + fence/hair/line
Becoming aware of collocations is an important part of vocabulary learning. In all languages, there are many words that naturally go together. A good dictionary can help you, and there are also dictionaries specifically focused on collocations.