In this specific line, the brook describes how it shapes the banks of its stream. The verb "fret" here means "to erode or wear away by rubbing or friction". So, the brook suggests that its continuous flow and movement cause it to create curves and bends in the banks along its path.
By using the word "fret", Tennyson gives a sense of the dynamic nature of the brook. It implies that the brook is not a static body of water but is constantly shaping and reshaping its surroundings. The brook actively interacts with its environment, leaving a mark on the landscape through its erosive power.