Ago has 6 different meanings across 4 categories:
Adjective · Adverb · Postp · Proper Noun
Gone; gone by; gone away; passed; passed away.
"The old king died many years ago, and his reign is now just a memory of what has passed."
In plain English: Ago is used to say how much time has passed since something happened before now.
"There is no adjective form for the word ago, so I have written an example using its standard adverbial usage: We left two hours ago."
before
"The package arrived three days ago."
Before now.
"I visited my grandmother three days ago."
Initialism of Attorney General's Office.
"The acronym A.G.O. stands for Attorney General's Office, not a time period ago."
The word "ago" comes from Middle English and originally meant something that had passed or departed. It traveled into modern usage to describe the time elapsed since a specific event occurred.