When it's one o'clock it's one o'clock. It's time to eat lunch, or take a nap, or something.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
I eat lunch at one in the afternoon.
Can you email me by one o'clock?
It's not normal to visit someone's home at one in the morning.