These elastic straps attach to your rear rack and help you secure things to the top of your rack (a jacket or two, umbrella, small box, bike lock, pair of gloves, or purse, for instance). They're nearly taken for granted on account of them being a standard feature on most Dutch bikes. The straps attach down near the dropouts of the bike (where your rack would attach) and from their a trio of straps loop up and over the rack to the other side. You can get them here to replace old or worn out straps or to install them on a bike that doesn't have them (a rear rack is a prerequisite for these, if that's not self-evident).