Oh, I am sorry about that.  Can you please show me a list of all of the cities located in counties that for which the county has a population over 20,000?
SELECT  Name FROM city WHERE county_ID IN  ( SELECT county_ID FROM county_public_safety WHERE population  >  20000 ) 
list of all of the cities located in counties with a population over 20,000 refers to selecting cities based on a condition applied to their related counties; condition 'county has a population over 20,000' corresponds to WHERE population > 20000 in a subquery.