COMPARE:

Dummy IT

"IT seems John is sick."    ---   IT doesn't refer to a specific thing or entity, and thus can't be replaced by a word or words for what it refers to.

versus

Full Pronoun IT

"It is on the table."   ---   "It" can be replaced by a word that is context specific, e.g. the "book", "lamp" or even abstract/metaphorical references such as the "answer".