What is a subject pronoun?

They are words like I,you,he,she,it,we,you,they. In English they normally come before the verb to say who does the action.
In English we normally use a subject with a verb:

EG: I drink, you drink, he, she or it drinks, you drink, they drink.

In Spanish we don't always use the subject pronoun because with person who does something is contained inside the conjugation of the verb.

EG: Bebo = I drink - Bebes = You drink . Bebe = he,she or it drinks, Bebeis = you drink, Beben = they drink.

In Spanish you only need to use the subject pronoun for emphasis or when it is not clear.

Note: "Bebe" = he, she or it drinks.
If you say "bebe" it is not clear who drinks so you many want to specify this by using :-

él bebe = He drinks - "Ella Bebe" = She drinks - "Ello Bebe" = It drinks.