The subject in a sentence is "who" or "what" you are talking about. Every sentence needs a subject. If you don't have a subject, then the sentence is incorrect and nobody will understand what you are talking about.
In other languages, the subject is not always required. Verbally, the person listening to you will understand what you are talking about, so a subject is not required. In English, a subject is always required.
Here are examples of small sentences with the subject underlined.
"I am hungry"
"My brother is very smart"
"That computer is very expensive"
"We are going to the store now"
"My sister and I will be waiting here"
"The building is very big"
"When are you going to eat lunch?"
"Why are they waiting in line?"
"Who is going to take you to the store?"