MAO (monoamine oxidase) is an enzyme that breaks down various neurotransmitters (chemicals in your brain) -- specifically, serotonin, dopamine, and norepinephrine (and possibly some others). These chemicals - especially serotonin and dopamine -- make you feel good normally and when you do drugs, the release of these chemicals in your brain is what makes you feel high.
A MAOI is a MAO-inhibitor -- it prevents/limits MAO from breaking down the chemicals I mentioned above. Thus, for people who do not produce enough neurotransmitters and are depressed or have other psychiatric problems, taking a medication that is a MAOI helps keep the chemicals in their brains (in the synapses) longer, therefore making them happier.
The problem with combining many drugs with a MAOI is that a lot of drugs (e, meth, probably lsd and other psychedlics, and others) cause the release of large amounts of neurotransmitters in your brain, which can't be broken down because of the MAOI, and therefore overwhelm your brain. This can lead to serotonin syndrome, a hypertensive crisis, and even death -- a very dangerous combination indeed.