"I like roses."
Translation:J'aime les roses.
In french 'aimer' and 'adorer' and similar verbs are followed be a definite article (le, la, les). That's just the rule. It doesn't translate into english, it's just is a rule the language follows. 'Des' actually ends up being more specific because it would mean more like "I like some roses, but not others."
Generally speaking, these articles (known as partitive articles) are used when there would be a "some" in English ("I am eating some lemons" = "Je mange des citrons") or when there would be no article at all in English ("I am eating lemons" = "Je mange des citrons"). The correct one to use is based on the number and gender of the noun it is attached to:
"du" is a contraction of "de le" and used for masculine, singular nouns (e.g. "Je bois du lait")
"de la" is used for feminine, singular nouns (e.g. "Je mange de la viande")
"des" is used for plural nouns, both feminine and masculine (e.g. "Je veux des fruits" or "J'ai des filles")
"de" is sometimes used instead of the above if an adjective comes between it and the noun (e.g. "J'ai de gros problèmes"); it is also used instead of the above in most negative sentences (e.g. "Je ne mange pas de fruits")
With regard to this particular example, as heydarling noted above, verbs that have to with liking or disliking (such as "aimer," as used here,"adorer," "préférer," "détester," etc.) don't use the above. Instead, they use the definite article (le/la/les) such as in this example.
One more thing for future reference to prevent any confusion: sometimes du/de la/des is not a partitive article, but rather a combination of "de" (meaning "of") and a definite article (le/la/les). For example, "the girls' books" is "les livres des filles" (literally, "the books of the girls").
"Adorer" is stronger than "aimer," so it would be used for talking about something you really like or love. If you simply like something, as is expressed here, "aimer" is the accurate word to use.
(The reverse is true if you're talking about people, with "aimer" having a stronger meaning, indicating you love someone (at least when used without qualifying adverbs such as "beaucoup" or "bien," and "adorer" indicating you quite like someone as a friend.)