Sentences in English may be in active or passive form. The voice of a verb is a form or set of forms of a verb showing the relation of the subject to the action.
The potential is used to express that one has the ability to do something.
The potential mood is used to indicate an action that is likely to happen.
I read Japanese.
I can read Japanese.
I eat fish.
I can't fish.
It is also used to request some action from someone, in the exact sense of the English "Can you ... ?" "Can (you) buy (some) coffee?" However, sometimes in English "Will you...?" and "Can you ... ?" is used interchangeably to make requests.