What does the idiom many moons ago mean?

Answer 1

Before there were calendars in everyone's possesion, people referred to the phases of the moon. Each time the moon goes from First quarter, full, last quarter, back to new moon, is a "moon". This process takes about a month. So saying "many moons" just means "many months".

I guess people can still say it, but it was much more common when people used the phases of the moon to signify a month instead of looking at a calendar.