A Choropleth map is a thematic map visualization, which allows you to mark areas (in Redash's case, it's Countries) according to specific values.
The countries are patterned according to the designated value (column), thus providing a convenient way to track distributions per country.
For example, the following Choropleth map shows the number of Redash users in different countries:
Note that the minimal unit in a Choropleth is a Country. Currently, it's impossible to drill down into smaller areas.