A euphemism for the anus, "b hole" is simply an abbreviated way to say "butt hole."
"So I was hanging out with Stacy last night and she started telling me she wanted it in her b hole, but I'm not into that so I just left."
"Man, their burritos are so spicy! Seriously, get their fuego burrito, last time I had one I ended up putting blister cream on my b hole."
"Dude, your cat has the biggest b hole I've ever seen."