means A significant point or achievement in the progress, development, or history of something.

from Literally what it says: a stone set by the roadside marking the distance in miles. The Romans were obsessive about this, planting milestones (milliarium) along their vast network of roads so travelers always knew how far they'd come — and Rome itself counted from a golden 'milliarium aureum' in the Forum. English 'mile' descends from Latin 'mille passus,' a thousand paces, so a milestone was the thousand-paces marker. The figurative sense — a landmark moment in a life or project — grew naturally from the road: you pass a marker, you note how far you've traveled, and somewhere along the way the journey stopped being about literal distance.