Ago means in the past, counting backward from a particular moment. When you say something happened five minutes ago, you mean five minutes before now. When your grandmother talks about something that happened sixty years ago, she's measuring back six decades from today.

The word usually looks backward in time from whenever you're speaking. If you read a book written two hundred years ago, that means two centuries before the present day. A dinosaur that lived 65 million years ago existed that long before the present day.

Notice that ago only works when counting from a specific point in time. You wouldn't say “I arrived five minutes ago when you got here.” Instead, you'd say “I arrived five minutes before you got here.” The word ago uses a reference point in time as its starting point. It's like having an invisible clock that only measures how far back something was, never forward.