When we use the words it and there to begin a sentence without a referent (a noun the pronoun is referring to), we’re using a dummy subject.
In this pair of sentences:
I went to see Fantastic Four 2 over the weekend. It was fun, but mostly forgettable.
“It” refers to the movie Fantastic Four 2. The pronoun has a referent.
In this sentence, however:
It is apparent that oil reserves will be exhausted by 2050.
“It” has no referent, and is therefore a dummy subject.
The same thing happens frequently with there:
There are several ways in which you could begin.
There are five stages of grief.
Dummy subjects are just one of many problems that weaken your writing by making it vague, fuzzy, and indefinite. The sentences above can be reconstructed with stronger, more definite subjects:
Some experts warn that our oil reserves will be exhausted by 2050.
You could begin in one of the following ways: (followed by a list).
Elisabeth Kubler-Ross identified five stages of grief in her book On Death and Dying.
In general, unless you don’t know who is performing an action, or you want to emphasize the action of the sentence for some reason, you should avoid dummy subjects.