lt turns the tultle left

lt pivots the clockwise (Left Turn).

The turn is measured in degrees.

lt 90

Notice that the tultle will not actually move in the new direction until you tell it to move forward with fd.

When using a pen, putting lt between two fd moves can draw an angle.

pen purple
fd 100
lt 120
fd 100

Notice that the angle used by lt is the exterior angle. The interior angle added to the exterior angle will be 180. For example, the 60 degree interior angle drawn by the program above corresponds to the 120 degree exterior angle swept out by the tultle.

lt can also be used to draw curves by using a second argument. Read the reference page on curves.