It will assign the return value of canvas.getContext('2d') to ctx if canvas.getContext is actually a function.
The part on the left is just to avoid an error. It makes sure that canvas has a getContext property before trying to call getContext(). This way, the code won't call canvas.getContext() if the function doesn't exist. If it didn't check first and called when the function didn't exist, an error would be logged to the console and execution would halt.
