It refers to BTS' fandom called ARMY(Adorable Representative M.C. for Youth). BTS considers ARMY as their "Blood, Sweat and Tears", their "Universe", their "Wings" and their "BIGGEST VOICE".
If you're an army, consider yourself as someone's biggest voice.
ARMY is our BIGGEST VOICE for we are voiceless alone. We are weak but with them, we are together, bulletproof.