The zero milestone is a special point in certain cities that marks the beginning of the city's road network. It's kind of like a starting point for all the roads in the city. Picture a big red dot on a map and that's the zero milestone!
It's usually a big stone monument or a metal plate in the ground that has the number "0" on it. The zero milestone is often a special spot because it is the point from which distances are measured in the city. So if you ever walk by a sign that says something like "3 miles to downtown," that distance is measured from the zero milestone.
In other words, the zero milestone is an important spot that helps people understand where they are in the city and how far away they are from other places. It's like a giant compass in the middle of town!