Ago and back are used for past times from the present moment:
▶️ I graduated from high school ten years ago.
(ten years in the past from today)
▶️ We sent the package three days ago.
(three days in the past from today)
▶️ I moved here about five years back.
(informal – five years in the past from today)
Before and earlier are used for past times from another time in the past. Here are
some examples:
▶️ Yesterday I missed my train. I got to the train station at 7:10, but the train had
left ten minutes before. (or ten minutes earlier)
(= ten minutes before 7:10 yesterday)
▶️ I was very happy when I got this job last January, because I had lost my
previous job six months before. (or six months earlier).
(= six months before last January)