ago is an adverb used to indicate that something happened in the past, measured from the present moment.
For example: "I graduated from university 5 years ago."
Key points about ago:
1. Always used with a time expression (e.g. minutes, hours, days, years)
2. Comes after the time expression
3. Refers to a specific point in time, not a duration
4. Commonly confused with "before" and "since"