What does “meth“ mean in “Meth heads snuck in to it to do meth.”?

There was the following passage in the New Yorker’s (September 22, 2014) article titled “Field trip” describing a photographer who travels to take pictures of oldest living things in the world:

The one she photographed was a 3500 year-old tree just outside Orlando, Florida – actually the original tourist attraction before Disney. Meth heads snuck into it to do meth, and they accidentally burned it down. One of them later said, ‘Oh shit, we killed something that was older than Jesus.’

As I had no idea about ‘meth,’ I checked Readers English Japanese Dictionary at hand, which defines it as a slang to mean methyl alcohol or drug, methadone.

Urban Dictionary defines meth as an abbreviation for methamphetamine, a drug that stimulates the central nervous system by causing it to release more dopamine, a neurotransmitter that gives someone a feeling of satisfaction.

Does ‘meth’ in the above quote means methadone or methamphetamine, and 'do meth' mean to take or inhale it? Is it a popular slang as pot and weed are for marijuana?

Meth (or sometimes crystal meth) means methamphetamine. It is the most common name for the drug, at least in the US. A meth head is someone who uses meth. Most people who understand pot or weed would understand meth. In exactly the same way that a person would "do heroin" or "do coke" one could "do meth."