"The apples are red."
Translation:Les pommes sont rouges.
"The apples are red" means all the apples we are talking about are red. >> "Les pommes sont rouges." "Des pommes sont rouges" is not a good phrase, but for sake of argument: it means "some of the apples are red." Let's invent a new context: "I have the red apples and not the green ones." >> "J'ai les pommes rouges et pas les verts." OR "I took some red apples" >> "j'ai pris des pommes rouges."
Des is a conjugation of de-les not de la because the noun is plural, so des roughly translates to 'some of the' or more so 'some' - where les is just 'the'.
since THE is LA, LE or LES so in this case LES, this sentence is talking about specific apples, so if it were more specific 'the apples (over there) are red' you use THE (les) apples are red.
if you were talking about apples in general, or no specific ones you would say 'apples are red', or 'some apples are red', since those statements are talking about a non specific set of apples you use DES.
Les pommes sont rouges = THE apples are red Des pommes sont rouges = apples are red -or- (some) apples are red