The monitor command prints timing (and other) information. In particular, it prints the number of lines and polygons generated, the time it took to render the plot, and the total time it took for the plot to both generate and render the plot. Please note that most plots do not supply the number of lines and polygons generated, so these two statistics are usually set to zero.
Each invocation of this command toggles the state. Originally, timing information isn't printed, so the first invocation of monitor turns on such printing. The second invocation turns it off again, and so forth.