It is snowing.
In the above sentence, the subject it is not clearly defined and does not show what it represents. In such cases when the use of it does not define what it really stands for, the verb is termed as impersonal.
It is getting dark.
In the above example getting is the impersonal verb. As it uses the subject it and does not really define what it really means or represents. Despite this, the meaning of the sentence is clear. This type of verb is called impersonal verb.