What do the little flags on these martian wind speed data points mean?
They indicate wind direction (given by the long line or "barb") and speed (given by the shorter flags extending from each barb). In the same linked page you can read:


  The barbs extending from each wind speed data point indicate the
  compass direction of the wind (e.g., a wind blowing from the north
  will have a barb straight up above the point; a wind blowing from the
  west will have a barb off to the left). Full and half flags extending
  from the barbs indicate the wind speed, with each half flag
  representing approximately 2.5 meters per second. A circle in place of
  a barb indicates a wind speed less than 2.5 meters per second.