BK7 is the type of glass the primary mirror is made of. There are various types of glass used in mirror making. Pyrex is another and considered low expansion. Which I'm lead to belive means it doesn't exspand or shrink as much due to temperture change while the scope cools to the outside temperture.

I'm sure others can explain it better than me, but I think thats the general gist of the matter.

Regards