BHO or ‘Butane Hash Oil’ is a concentrated cannabis extract. It is made by pushing liquid butane through a tube packed with weed. The result is a potent wax, oil, or honey.

BHO stands for Butane Hash Oil

It is also sometimes called “Butane Honey Oil.” The reasoning works like this: Cannabis has trichomes, hairy projections on the outside of the flower, in which a good deal of the concentrated THC, CBD, and other compounds we want is contained. So they stuff bud into a tube, pipe butane through it, and use that as a rinse so those compounds away from the flower and into the butane mixture. They then take the butane mixture which contains the oil from the cannabis, and purge the butane out of it using vacuum technology, leaving the cannabis extract.

Here is a lightweight Vice overview on the subject:

EVERYTHING you’ve heard about the dangers of BHO extraction is true!

Do not try this at home! The issue is that butane, even though it makes the ideal solvent for blasting through weed, is chemically unstable, highly flammable, and volatile to work with. Butane is literally the liquid they use in cigarette lighters, it’s that ideally flammable. A single spark in the same room with a bowl of butane can cause an explosion. Somebody flips on the light switch, boom.

BHO extraction needs to be done by trained, practiced professionals who know what they’re doing. Even the pros goof up sometimes, because the equipment can fail or an accident can happen.

• What Is BHO Wax and How Is It Made?
• BHO Wax Continued. Dangers, Impurities and How to Know What’s Good

It is also illegal to extract BHO at home, even in legal states

There are strict laws controlling how cannabis concentrates can be produced in any state which legalizes cannabis for medical or recreational use. Most states ban volatile extraction methods outright. Some allow it under licensed circumstances. California, for example, has separate licenses for extraction specific to whether or not it’s done with volatile solvents.

There are plenty of alternatives to BHO extraction that are nearly as good, without the risk

Our HOWTO section covers a wide variety of methods for making your own vape oil, edible oil, and rosin.