We use 'ago' to talk about time in the past from now. It tells how long ago something happened.
I saw her two days ago.
The movie ended five hours ago.
Rarely, 'ago' is used with a past reference point, meaning before that past moment happened.
By 1900, the castle was built decades ago.
She arrived a week ago from when you called.
'Ago' often appears in idioms meaning a very long time in the past, showing great distance in time.
That was centuries ago.
People lived differently ages ago.
Rarely, especially in poetry or old texts, 'ago' can mean the past time itself, acting as a noun.
They speak of the ago with great respect.
In that ago, life was hard.
