There is a big difference between a collocation and an idiom.
A collocation is the way words combine in a language to produce natural speech and writing. For example when you say "pay attention", it could be "give attention, or put attention" but it is not, it is pay attention because it is the natural way in which native speakers express that.
An idiom is a sequence of words which has different meaning as a group from the meaning they would have if you understood them separately. For example, when you say "it's raining cats and dogs" you do not really mean that cats and dogs are falling down from the sky but that there is a heavy rain.
I hope my answer is helpful.
A collocation is the way words combine in a language to produce natural speech and writing. For example when you say "pay attention", it could be "give attention, or put attention" but it is not, it is pay attention because it is the natural way in which native speakers express that.
An idiom is a sequence of words which has different meaning as a group from the meaning they would have if you understood them separately. For example, when you say "it's raining cats and dogs" you do not really mean that cats and dogs are falling down from the sky but that there is a heavy rain.
I hope my answer is helpful.
