This is one of Fear Inoculum's four interludes: brief songs bridging the (much) longer ones. The song is all distorted vocal effects and synthesizer sounds. There are faint words, but it's almost impossible to understand what they're saying (if anything).
The word "legion" was originally used for the major units of the Roman army, having up to 6,000 infantrymen and 200 cavalry. It has since become a common term for a large number of people in pretty much any context or capacity.
The word "inoculant" refers to the active material used in an inoculation, with inoculation meaning "the introduction of a pathogen or antigen into a living organism to stimulate the production of antibodies."
What Tool means by combining the two words is unclear, but it's probably something like "making the individual resistant to the masses" or maybe "making the individual resistant to the lure of joining the masses."
Legion is also the name claimed by a figure in the Bible. Mark 5:1-18 tells the story of a man living in the tombs and possessed by a demon. When Jesus, who is preparing to cast the demons out, asks the demon its name, it replies, "My name is Legion, for we are many."
New Order took the title for "Blue Monday" from an illustration, which read "Goodbye Blue Monday," in the Kurt Vonnegut book Breakfast Of Champions. The image referred to the invention of the washing machine improving housewives' lives.