There are two tables of data on the form, one for continents and the other for countries. Each country belongs to a continent and has the relevant continent id as part of its data.
The page has two repeater controls which are backed by the tables. Repeaters can display repeating data with any type of custom layout. You can put any control into the repeater, including another repeater. The COUNTRIES repeater is inside the CONTINENTS repeater.
The key to displaying only the related countries for each continent is the Filter Expression property on the COUNTRIES repeater control.
The Filter Expression is set to:
CONTINENT-ID = COUNTRY-CONTINENT_ID
This means "Only display countries that have the parent repeater's continent id".
Repeater controls are very useful for displaying data in a flexible way.