Learn Spanish (Latin America): Son las diez en punto.

In English, we always say it is for time. In Spanish, hours are treated as plural things. Because ten is more than one, we use the plural son (they are). You will only use the singular es for one o'clock (es la una).

Why do we say las diez instead of los diez?

The article is feminine (las) because it refers to the Spanish word for hours (las horas). Even though we leave the word horas out of the sentence, the article still has to match its gender.

What does en punto mean literally?

Literally, en punto means on point. In Spanish, it is the standard phrase used to say on the dot, exactly, or sharp when telling time.

Do I always have to add en punto when telling the time?

No, it is optional. You only need to add en punto if you really want to emphasize that it is exactly ten o'clock, with no extra minutes. Saying just son las diez (it is ten) is perfectly fine for everyday conversation.