The /clear command deletes all text from the chat window up to the point where you used the command, so that you no longer see it. This is often useful after playing for a long period of time to clear the chat buffer and free up some memory (possibly reducing lag on some systems). /clear will also remove all of the speech bubbles and other messages from the upper part of the screen so it is useful to "clear the view" when things get too cluttered to see the action on deck or on land, such as when a large number of pirates board a ship in quick succession.