It is the performer or the doer of the verb. In English every sentence must have at least one subject. The subject could take the following forms:

NOUN OR EQUIVALENT.

a) A subjective pronoun (I, you he, she, it, we, they)

They took the bus at 6:10 am.

I read a magazine everyday.

She combs her hair.

b) A Noun:

Peter is a good student.

Students are not only intellectual but also social and emotional beings.

c) A noun phrase:

The truck is in the middle of the road.

How to do that is difficult to decide.

The studious boy always gets good grades.

d) A noun clause:

What annoys me the most are those noisy parties.

I believe that he sings beautifully.

What Stephen did impressed the audience.

e) An infinitive phrase:

To walk is healthy.

To sing helps you develop your acting skills.

To protect the environment is fabulous.

F) A gerund phrase:

Walking is healthy.

Signing helps you develop your acting skills.

Protecting the environment is fabulous.

G) An adjective:

The rich don’t need more money.

The poor envy the rich.

The involved should turn up in the court.

The affected must confess their crime.

e) A verbal noun phrase:

The singing of the birds is awakened.

The reading of a book without reflection is useless.

The listening of classic music is relaxing to our ears.