This is the practice whereby couples affix a padlock with their names or initials to a prominent landmark, usually a bridge. It's sort of the modern equivalent of carving your initials into a tree.
It started somewhere in Europe (no one knows quite where) and spread worldwide.
I really like it. It's kind of sweet, but I think it looks good, too. Many people spend a good deal of time and effort picking out just the right lock. Some spend quite a bit of money to get a lock that will endure for years to come.
Put them all together and, as this picture I took on the pedestrian bridge over Ottawa's Rideau Canal shows, the whole thing has a great aesthetic.
And, I am pleased to say that, unlike many cities where the authorities have taken bolt cutters to the locks, the City of Ottawa is quite happy to leave them in place for all to enjoy.