I first learned about "bears" in Ptown, which apparently leans gay anyway. Had a weekend off while in MA for some work and was convinced to go there for a day trip. I got the warning on the way about what to expect, only this particular weekend included some big gayfest with among others, many hairy bikers. I several times heard people shout stuff like, "I'm here with my bear".
Once I put it all together, I made sure my shirt was buttoned up far enough not to expose my hairy chest, and I made Hot Ukrainian Coworker and the female LCDR we were with walk on either side of me as bodyguards the whole time.