In this sentence “un des” is like “one of”.

Here the meaning is:

France is one of the greatest countries in Western Europe.

(My english is still bad, sorry, but the meaning of “des” is correctly translated)

The sentence

La France est un des grands pays de l’Europe occientale

can be translated as

France is one of the major countries of Western Europe

un des is here used to say one of the

Pays is a word that has the same form in the singular and in the plural. It always has an s in the end.

In :

un des grands pays

it is plural. And you can tell it is plural because the adjective grands has an s, since in French adjectives agree with the nouns they modify.

Here des (standing for de les) is a preposition followed by the plural definite article.

If you wanted to say a great country it would be un grand pays with no s a the end of “grand”.

“Un(e) des …” is another way of saying “un(e) parmi les …“ (One among the, one of the…)