"des roses" is the plural of "une rose", ie an indefinite quantity of roses (more than one).
In English, you get this meaning with "roses" or "some roses".
"les roses" is specific with this action verb, they are those mentioned before or those the speaker can see right now.
In English, you get this meaning with "the roses"
"les roses" would be viewed as "general" in sentences where all roses are concerned, like:
- les roses sont des fleurs = roses are flowers
- j'aime les roses rouges mais je préfère les roses blanches = I like red roses but I prefer white roses.
In English, a general statement about "all roses" does not have an article.
"le cheval" is specific here, it was mentioned before or it is the one the speaker can see right now.
In English, a specific horse is "the horse".
"le cheval" can be made 'general' within a sentence that makes it general and not specific, but I think this would rather happen in plural, like:
- les chevaux sont les meilleurs amis de l'homme = horses are man's best friends.