A milestone is the indication of the population growth of a city. When reaching the goal of a milestone, a city can unlock new features, services, land areas, buildings, city policies, and much more.[1]
The population requirement is not analogous between each map as each have different population requirements. The requirements may be the result of the buildable area within a map.
All milestones may be unlocked instantly in the Content Manager panel > Mods
Note: This is old and doesn't include all DLCs and CCPs in the description.