It means somebody drew (or printed) a circle with a cross on it.To find out what it means on any specific map, you should consult the key and/or legend, which should tell you what each of the symbols mean.
A circle with a small cross inside represents a port on a map.
A star is a capital and a circle is a city
capital city
it means a capital
capital city
Compass Rose
Compass Rose
a state capital
A capitol city.
Compass Rose
it means the national capital on a map