Learn Spanish (Spain): Son las dos.

In Spanish, hours are treated as plural, so we use son for every hour except one o'clock. Since two is more than one, we say son las dos. You will only use es for one o'clock (es la una).

Why is the feminine plural word las used before dos?

The word las is used because it refers to the hidden word horas (hours), which is a feminine and plural noun in Spanish. You will always use la or las right before the number when telling the time.

Is there a Spanish word for "o'clock" in this sentence?

No, Spanish doesn't use a separate word for "o'clock." You simply state the hour using the correct form of the verb ser (to be) and the word la or las.

Why isn't there a word for "it" at the beginning of the sentence?

In English we say "It is...", but Spanish does not have a direct word for the subject "it". The "it" is simply built into the verb son.