The Imperial trading fleet are ships from Lord Richard Northburgh that will visit your harbours and trade goods with you. The fleet starts with one ship with a budget of 500 gold.
The trading fleet can be upgraded by buying the Imperial Merchant Fleet attainment, which when upgraded twice to the final level proviceds 3 ships with a budget of 2,000 gold per ship.
The trading fleet can't be attacked in any way. You can try normal fire, letter of marque, powder keg or boarding crew, but it doesn't work.
In the beginning these ships are very useful because you don't have enough ships yourself to export/import products. See the Trade page for more information.