The term collocation refers to conventional word combinations. They are usually easy to understand, but not so easy for a foreign learner to produce correctly. Some examples of collocations are given below:

A burning desire (BUT NOT a blazing desire)
A blazing row (BUT NOT a burning row)
A heavy smoker (BUT NOT a devoted smoker)
A devoted mother (BUT NOT a heavy mother)
Thanks a lot (BUT NOT Thank you a lot)
Change one’s mind (BUT NOT change one’s thoughts)
A golden opportunity (BUT NOT a golden chance)

Formation of collocations
Collocations are typically governed by conventions. In a sense they are idiomatic. You can, for example, think of many adjectives that can be used with the noun smoker to say that somebody smokes a lot. It just happens that English speakers use heavy, and not big, strong, fierce, hard or mad. A learner has to know these correct combinations in order to express the idea correctly.

A foreign learner who uses wrong combinations may still be understood, but he or she will not sound natural.

Situational language
By situational language we refer to those expressions that are typically used in everyday situations.

Examples are:
check the oil (But not usually ‘inspect the oil’)
Keep somebody waiting (More natural than ‘make somebody wait)
Is it a direct flight? (More natural than ‘Does the plane go straight there?’)