it stands for the uk band bring me the horizon. they use the initials on their merch and people sometime refer to them as simply BMTH

It stands for an amazing band names Bring Me The Horizon. Many fans refer to them as BMTH.
BMTH is a UK band, but they are well known in other countries including the US.
The band members include Oliver Sykes (vocals), Matt Nicholls (drums), Matthew Kean (bass), Lee Malia (guitar), Jona Weinhofen (guitar).

Bring me the horizon are a heavy metal band, deathcore, thrash, whatever you like. They are very talented for thier age, early twenties. People have this hilarious idea that its "cool" to hate them, well, let me kill all your little theories about them alright? Firstly, Oliver, as many of you despise, is not "some emo faggot" He is a regular guy who doesn't give a shit what people think of him, having met people who are very close friends with him and having met him myself, he is a kind down to earth person. And he pissed on someone? That is, sheer bollocks. Right. I saw on a fucking interview, he said he would be wasting his time trying to justify himself to thousands of people - and at the end of the day people can think what they like - but to be fair, there was not enough evidence to press charges on him - doesnt that tell you something? Law is stronger now days and lack of evidence means some stupid little whore wanted attention and no doubt made up the storie but couldnt back it up - further, thier song "No need for introductions I've already read about girls like you on the back of toilet doors" says his message loud and clear. And, if you think thier fans are only "14 yr old girls" then why do you see huge great men wearing dropdead into heavy metal at thier gigs? They have more male fans than females, i would know. And if theyre so shit and unpoplular then please explain why they are touring the entire world and selling out entire shows? Untalented? hardly... again they wouldnt be as succesfull as they are if they were untalented, and theyre young anyway - but yes, Oli can scream well, no matter what anyone says - he has skill; and yes, some stupid little 14 year olds probably do only like the band for oli and his good looks, but at the end of the day, its the same with ALOT (and i mean ALOT) of other bands - the blackout for one. And how does that effect anyone? let the silly little girls drool - whilst thier real fans mosh til they break something at thier crazy arse shows, to them its about the music and they said in another interview they obviously do care how they look, but they didnt start the band to show off thier haircuts they started it because of MUSIC. So, there you go. Bring me the horizon are GOOD. get used to it.