It's vapor that has condensed on the glass. Think of it as vapor your vape has saved for a rainy day . If you soak the bowl in alcohol (maybe add some salt) in a glass or ceramic bowl for a day, a lot of the residue will come off. Swish it around a few times during the soak. When you take it out and finish cleaning with a swab and pipe cleaner, let the alcohol evaporate and the residue that's left can be scraped off and smoked or vaped. It may be gummy or dried to the point where it scrapes off in flakes. Up to you whether you save it or not, but it definitely has potency. It's not at all like smoking the black resin left in a pipe.