"Leftward" isn't commonly used, but it basically means, "in the direction to your left" . Viz. toward, forward, backward, sideward, upward, etc... these are all directions. "Left side" is a location.

Thank you so much for your comment!! I am interested in the topics! It is just Wild west days.　All Japanese study history of United States class in history when they were school days. However,it is a little. Please teach me many things of United States☆

This reminds me of "westward". A long time ago in the United States ( when the pioneers traveled west from the east in their wagons with horses) the leader of a wagon train group said: "Westward, ho!" This meant, let's start traveling now ---to the west.