The monitor is a heavily armored steam-powered military ship capable to carry a low amount of cargo. It is armed with a heavy mid-range turret.
The Monitor is a tool for late-game naval dominance, and not just because it takes Investors to unlock it. While it lacks the range of the Battle Cruiser, a nation can just about have three Monitors for every one Battle Cruiser. It also pairs up much better with the Battle Cruiser than any sailing ship as both can ignore the wind, and both have turrets. The Monitor then is the Frigate equivalent to the Battle Cruiser's Ship-of-the-line--faster, cheaper to run, and well capable of overwhelming an enemy.
The original Monitor was a Union ironclad warship built during the Civil War, as a coastal warship armed with 11-inch guns to fight the Confederate ironclad Virginia. It was the first ship to use a turret in combat, and would inspire numerous follow-ons throughout the war. As a result, the name "monitor" would hence come to refer to small coastal ships carrying outsized guns.
The Monitor in Anno 1800 appears to use elements from both the Union and Confederate schools of design--the overall shape of the hull resembles the Virginia and its descendents, but the turret is distinctly a Monitor-style turret.   
Union Update 2 (May 15, 2019):
- Hitpoints buffed from 2500 to 3000
- Upkeep buffed from -600 to -300.
Game Update 5 (September 10, 2019):