Guten Morgen!

... in the morning you say...
Guten Morgen!
Good Morning! Good Day! ... Hello!
It means literally: good morning. "Guten" means good, while "Morgen" is the morning.

Guten Tag !

which is the expression for Good afternoon in German. ...from 11 am on you are safe to greet with Guten Tag. You will often hear only "Tag" which is very casual and popular.

Guten Abend!

means...
Good evening in German!
You start saying ’Good evening’ as way to say hello in German around about 7 p.m.-.. but also in that case there is no typical cut-off point.
...and "Guten Abend" is not only a way to say Hello in German, i.e. as a greeting, but you will also hear it as a farewell.

Also used is ... Hallo!

Hello! ... Hi! Hallo is more informal, and it can be used at any time of day. It is safe to use it, but be aware that it is a casual greeting.