This is the 115th most frequent Ukrainian word.
бойових
"Combat" or "military" (adjective).
In this sentence, "бойових" is used adjectivally to modify the noun "мистецтв", translating to "martial" and indicating the kind or category of the arts discussed.
Ми вивчили всі аспекти бойових мистецтв.
We studied all aspects of martial arts.
Here, "бойових" modifies "коней", meaning "war" and describing the type of horses which are specialized for combat purposes.
Бойових коней готували для походу.
The war horses were prepared for the campaign.
In this usage, "бойових" describes "героїв", meaning "battle" or "combative" and adding the context of their deeds or roles in history.