一人 (hitori) means “alone, one person”

More examples

教室に一人います。kyō shitsu ni hito ri i ma su

There is one person in the classroom.

週末は一人で映画を見ます。shū matsu wa hito ri de ei ga o mi ma su

I watch movies alone on weekends.

It presents one human being as separate from a group, which gives it both a counting sense and an unaccompanied sense.

個人kojin: One person considered separately from a group or organization.
単身tanshin: A person who lives or travels without a spouse or family.

人の数が一であること。また、ただ一人で他の人がいないこと。hito no kazu ga ichi de a ru ko to ma ta ta da hito ri de hoka no hito ga i na i ko to

一人 (hitori) is written with 一 (ichi, "one") and 人 (hito, "person"), and their combination has the irregular native reading hitori.

The people counter usually uses 人 (nin), but the common forms 一人 (hitori) and 二人 (futari) are special exceptions.