It means that the MAOI will simply detach from the MAO after a while, leaving it as it found it, working normally. A non-reversible version will destroy the enzyme, forcing the body to produce more to function normally.

the stuff in most plants people are eating here is reversable. irreversable MAOI's are much more dangerous and only useful for serious medical conditions or pharmacological studies as far as i know. i think the older synthetic ones were irreversable.