Improve the look and durability of wood siding
The wood appears solid, near as I can tell, and I don't see signs of peeling of the old finish. I'd simply apply a quality siding/fence stain that's tinted dark enough to mask the varying hues in the board and that coordinates with your home's exterior. I wouldn't attempt to scrape it due to the likelihood of damaging the grain, but maybe hit it gently with a nylon bristle brush to remove any loose material. 

Note that "stain" is a broad term, and by specifying a siding/fence stain I'm implying that the product is also a sealer. Interior stains, by contrast, are not intended as a sealer or finish coat.