Learn Hardcore Indonesian: Dia sakit hari ini.

Dia is a gender‑neutral third‑person singular pronoun. It can mean he, she, or sometimes they (for a single person of unspecified gender) in English.
Indonesian usually relies on context (who you were just talking about) to clarify gender. If you really need to specify, you normally add extra information, for example:

Dia laki-laki. – He (is) male.
Dia perempuan. – She (is) female.

On its own, dia sakit hari ini is ambiguous: it could be He is sick today or She is sick today.

Why is there no word for is in Dia sakit hari ini?

Indonesian does not need a separate verb like is/am/are before adjectives. Adjectives can function directly as predicates.

Dia sakit. – He/She is sick.
Dia lapar. – He/She is hungry.
Dia capek. – He/She is tired.

So in Dia sakit hari ini, sakit already serves as the predicate is sick, and you simply don’t add anything like is.

What exactly does sakit mean? Is it sick, ill, or in pain?