The map is an Ocean Chart owned by the Bellman, one of the key characters in the story. It helps the Bellman and his fellow adventurers, who are hunting for a legendary beast called the Snark, to cross the ocean and arrive at a strange land. The absurdity of the map is that it only shows ocean - literally illustrating nothing, and therefore supposedly not a very helpful navigating tool. Here’s an extract from ‘Snark’ relating to said map:
He had bought a large map representing the sea,You gotta love it - it's very much a map for our world... the normal landmarks of societal structure simply don't make sense in the way they did for previous generations... we are into uncharted, unmapped territory.
Without the least vestige of land:
And the crew were much pleased when they found it to be
A map they could all understand.
“What’s the good of Mercator’s North Poles and Equators,
Tropics, Zones, and Meridian Lines?”
So the Bellman would cry: and the crew would reply
“They are merely conventional signs!
“Other maps are such shapes, with their islands and capes!
But we’ve got our brave Captain to thank:
So the crew would protest) “that he’s bought us the best—
A perfect and absolute blank!”