REGISTER & VOICE
We are all generally very adept at choosing a suitable style of language to suit different social occasions. For example, talking to a friend, we choose a different style from talking to mum or dad or when talking to a doctor; and, we choose a different style when talking to a young child, and so on.
Sometimes a very formal style is called for, sometimes an informal style, sometimes a conversational or colloquial style will do and sometimes a friendly style, a terse style, an elevated style... and so on. The term register is used to describe these different voices or styles when the style is typical of a use of language in a particular situation or context. Written or printed language is similar, so we can talk of a 'tabloid newspaper register', or a 'legalistic register' and so forth.
In your own writing, your examiner or teacher will award you extra marks according to how well you have written in a suitable register or voice that suits the particular form, audience and purpose of the writing you have been asked to do.