who thought they were only mad when Baltimore gleamed in supernatural ecstasy,
Even after all this drinking of turpentine, jumping off buildings, and talking for seventy hours straight, they didn't realize they were "mad" (or insane) until they saw the gleaming lights of the city of Baltimore.
who jumped in limousines with the Chinaman of Oklahoma on the impulse of winter midnight street light smalltown rain,
They got into a limousine with a Chinese man in Oklahoma because it was cold and raining at midnight.
who lounged hungry and lonesome through Houston seeking jazz or sex or soup, and followed the brilliant Spaniard to converse about America and Eternity, a hopeless task, and so took ship to Africa,
This section of the poem concerns traveling, both through the United States and around the world.
They wandered around Houston, Texas in the hopes of finding good music, or someone to have sex with, or something to eat, or maybe all of the above.
As to who the "brilliant Spaniard" is, we haven't a clue. One of the early American explorers, perhaps?
But they found it impossible to talk about "America and Eternity" with this Spaniard, so they sail to Africa.
who disappeared into the volcanoes of Mexico leaving behind nothing but the shadow of dungarees and the lava and ash of poetry scattered in fire place Chicago,
This sounds like quite the trip. They disappeared into a Mexican volcano, leaving behind only the "shadow" of their pants ("dungarees") and the burnt-up fragments of poetry that they threw in a fire in Chicago.
who reappeared on the West Coast investigating the F.B.I. in beards and shorts with big pacifist eyes sexy in their dark skin passing out incomprehensible leaflets,
They're back in the U.S. again. Having started out in New York, they are now on the West Coast. They turned the tables on the F.B.I. by investigating it. (Which is like threatening to arrest a police officer.)
The speaker is describing the origins of the modern anti-war movement, which began in places like Berkeley, California with pacifists handing out leaflets to passersby on the street.