Learn Russian: Я вижу игроков, которым тренер советует отдохнуть. - I see the players the coach advises to rest.

In Russian, the verb советовать (to advise) requires the Dative case for the person receiving the advice, because you literally advise to someone. Since the coach is advising the players, the relative pronoun который must take the Dative form которым.

Why is it которым (plural Dative) when the word it refers to, игроков, is in the Accusative case?

The relative pronoun который takes its number and gender from the noun it refers back to (plural, for игроков), but it takes its case from its job in its own part of the sentence. In the main clause, the players are the direct object being seen (Accusative). But in the second clause, they are the ones receiving advice (Dative).

Wait, isn't -ов a Genitive plural ending? Why is игроков used after вижу (I see)?

You are right that -ов is a Genitive plural ending, but remember the rule for animate nouns! For living people and animals in the plural, the Accusative case looks exactly the same as the Genitive case. Since игрок (player) is animate and is the direct object of the verb видеть (to see), it becomes игроков.

Why doesn't the English translation include a word for которым?