Hello!

Usual greeting phrase is differrent form time of the day like in English.

In the morning,"Ohayou-gozaimasu"; Good morning
In the afternoon,"Kon-nichiwa";Good afternoon
In the evening,"Konbanwa";Good evening

Whenever you meet Japanese people, if you say"Kon-nichiwa",every Japanese people understands that you are saying a kind of a salutation phrase. And then,he or she says "Kon-nichiwa", back to yoo too.

Salutation words/phrases are the beginning of the conversation with people.
This is not a special feature of Japanese language world but common costom in the world.
Japanese people are not eccentric people.

Ohayou means "early".
But Japanese people are not sure the meanings of the phrase when they say it.Just knowing it as a phrase in the morning.

Think when you say "Good morning" to some people, are you consious that "It is good morning today!"?

The most typical phrase is a salutation phrase. Don't think, just feel. Fundamentals of the human is not so different among the people in the world.