A synonym is a word which means the same thing, or almost the same thing, as another word. A synonym for the word "happy," for example, might be "cheerful," "joyful," or "glad." In the first three lines of "The Chambered Nautilus," the speaker describes a "ship of pearl." A "ship" is a synonym for a boat. A "boat" is a generic term for a variety of vehicles that travel over water.
In the second line of the poem, the speaker says that this ship "Sails the unshadowed main." A "main" is sometimes a synonym for the hull of a ship, and sometimes it might serve as an abbreviation of "mainsail" and thus refer to the sail of the ship. In this line, however, the "main" could also be a reference to the ocean.
In the third line of the poem, the speaker refers to the ship as the "venturous bark." The word "bark" here serves as a synonym for the word "ship" or "boat." A "bark" is a ship with a square rig and three or more masts. The word "bark" also suggests the bark of a tree and thus wood, implying that the boat is an old, wooden boat.
The phrases "ship of pearl" and "venturous bark" both suggest that the ship or boat being described in the poem is precious, valuable, and adventurous. By describing the boat in these terms, the speaker sets the tone for an epic tale in the rest of the poem.