This is the 155th most frequent Czech word.
zemí
The Czech word "zemí" means "country" or "land" (genitive or instrumental singular, or nominative/accusative plural of "země").
Here, 'zemí' is used as the plural genitive case of 'země', meaning countries.
Navštívil jsem mnoho zemí během svého cestování.
I have visited many countries during my travels.
In this sentence, 'zemí' is the plural genitive case of 'země', referencing countries as parts of continents.
Mapa na stěně zobrazuje všech šest kontinentů a jejich zemí.
The map on the wall shows all six continents and their countries.
Here, 'zemí' is used as the singular instrumental case of 'země', meaning the land or soil, used in the sense of exploring the land.