A To understand this lesson is easy.
B It is easy to understand this lesson.
In this pattern, it has no meaning. It is used only to fill the subject position in the sentence. Thus, it is called introductory “it”.
A and B mean the same thing, but sentence B is more common and useful than A. A was introduce mainly to make the meaning of C easier to understand.
Introductory “it” can fill the position both of the subject and object.
Introductory “it” as a subject:
To watch musical programs is pleasant.
It is pleasant to watch musical program.
To play football must be fun.
It is fun to play football.